There are few online tools which allows you to analyze competitor ad space. I would suggest 2 tools SEMrush.com and Adgooroo.com.Both tools will provide very specific information about their organic search, paid search, backlinks, display advertising, top keywords, and main competitors.

There are various way you can promote with small budget but again it depends on various factors which depend on what you want to get out of it. If you are looking to increase your brand awareness than a quick solution is to have display ads on Facebook and Adwords with targeted segment.

If you are interested in discussing more about it than feel free to book for a call to have more personalized solution.

Similarweb.com is another useful tool to see what competitors are doing.

I was helping a guy in the used cellphone resale niche for a day, and he was initially convinced a social media campaign was the next marketing step.

A check of the top three competitors on similarweb showed they are getting less then 1/2 of 1% of their referrals from social media. So this evidence quickly changed his thinking about what would be effective in a marketing plan.

There are many resources for finding out where others are advertising. As for the most cost-effective tool, that's dependent upon what you offer and your target audience. If it's for products, consider Google Shopping.

Note that any advertising is an investment. If you have zero budget, consider other ways to build capital first so you can re-invest it. You'll find that you need to spend a decent amount on advertising first just to gauge exactly how you should be presenting content and bidding.

for Search (Adwords), SEMRush or Spyfu is good. I personally prefer Spyfu these days.
On display advertising, we use www.whatrunswhere.com. They offer a free 3 day trial, and then it's like $300/month, so has to be worth your time.

Would love to schedule a call if you're struggling with researching your competition.

The first answer to the question where your competitors advertising are the following websites that would immensely help you:
1. iSpionage: With iSpionage, you can enter a keyword or domain name in the search bar. From there, you will get competitor data and gain insights that will help you understand your competition’s strategy. You can get up to seven years’ worth of PPC data!
iSpionage compiles this data from three search engines — Google, Bing, and Yahoo. You will get data on your competitors and access to keywords, ad copy variations, and more.
When you use iSpionage, you will:
a. Learn from your competitors: With iSpionage, you can see who you are in competition with for Google Ads. This tool enables you to see your competition’s approximate budget for PPC ads, plus their strategies. It allows you to learn from their successes so that you can improve your campaigns.
b. Find better keywords: This tool will help you see what keywords your competitors use in their campaigns. You will get a list of keywords that you can sort by effectiveness to determine which keywords will be most profitable for your business.
c. Improve your click-through rate (CTR): With this tool, you can identify the competitor ad copy that is profitable. This information will help you create better ad copy so you can earn a higher CTR.
If you are looking to use your competitors as inspiration to improve your ad campaigns, you can use iSpionage to get the data you need.
Cost: $59-$299/month, plus custom solutions
2. SpyFu: SpyFu is one of the most effective competitor analysis tools for PPC campaigns. With this tool, you can learn from the ads that your competitors have tried and tested over the years. It provides you with valuable insight into what works and what does not.
With this tool, you can get insights into your competitors’ keyword strategy, and see any of your competitors’ keywords and ad spend history. It allows you to identify types of ad content that work so that you can integrate those strategies into your own campaign.
Spyfu enables you to:
a. See your competition’s estimated monthly budget: With Spyfu’s software, you can see how much your competition spends on their ad campaigns. It allows you to see how your stack up against these companies and can help you determine if you need to raise your ad budget.
b. Analyze every keyword your competition used: Spyfu allows you to go in-depth and learn about every keyword your competition ever bids on for ads. This information can help you better understand your competition’s strategy and inspire your campaign.
c. See how your competitor’s ad copy evolves: When you use this tool to monitor competitor ads, you can see how a company’s ad copy evolves for the same key terms. It provides you with insight into how competitors adapt their campaigns and can help guide you on how to develop your campaign.
If you want to learn more about your competitor’s ad campaigns, Spyfu will help you get the information you need about your competition’s advertising strategies.
Cost: $33-$299/month
3. AdLeap: If you want to know where your competitors are advertising, AdLeap can help. AdLeap allows you to get information about your competition’s Facebook ads and help you find high converting ads that your competitors run.
With AdLeap, you can:
a. Pin competitor ads: AdLeap allows you to pin competitor ads to your dashboard. You look through a library of ads and pick out ones that are similar to your business’s style and feature similar products. Pinning these ads enables you to gain information about competitor ad copy, images, and more, so you can use them as inspiration for your campaigns.
b. Create inspired ads: Once you pin different ad campaigns, you create your ad based on inspiration from your competitor ads. You get to keep your ad inspirations and your Facebook ads all in one place.
c. Monitor your competition: With AdLeap, you can keep tabs on your competition’s ads. You can search for competitors in your niche and track their ad progression.
If you want to monitor where your competitors are advertising and track their ad campaigns, AdLeap provides you with the resources you need to do so.
Cost: $47 per month for three months or one-time $97 charge
4. SEMrush: If you find yourself asking, “where are my competitors advertising,” you may need to use SEMrush to get answers. SEMrush provides information about both your search ad and display ad campaigns.
With SEMrush, you can analyse your competitors’ budgets and keyword selection. You can see what your competition is doing and use that as inspiration to improve your campaigns.
SEMrush will enable you to:
a. Monitor competitor ad copy: With SEMrush, you can get access to ad copy examples from your competition. This information allows you to see what calls to action (CTAs) your competitors use, the ad content, and more. It will help you create more effective ad copy.
b. Learn about new competitors: You may know some companies that you’re competing against for ad space, but do you know all of them? With SEMrush, you will get a list of competitors and the number of common keywords they use. It will help you identify who’s using the same keywords as you, so you know who you’re competing against for those key terms.
c. See what keywords your competition bids on: SEMrush allows you to see which keywords catch your competitor’s eye. You will see which keywords they’re bidding on and how much they’re paying for each click. It is a great way to help you shape your keyword strategy while tracking your competition.
SEMrush can help you understand your competition’s strategies better so you can create a more effective ad campaign.
Cost: $99-$399 per month if billed monthly, $83-$333 per month if billed annually
5. WhatRunsWhere: If you want to monitor competitor ads, you can use WhatRunsWhere to do it. This tool enables you to stay up to date on what your competitors are doing with their advertising campaigns. You can see top ads in your niche and discover what strategies your competition uses to win top ad spots.
WhatRunsWhere enables you to:
a. Identify winning campaigns: With this tool, you’ll see what competitor ads perform best. This information will allow you to identify what parts of their campaigns work and determine how you can incorporate similar elements into your campaigns.
b. Stay up to date on your competitor’s strategies: WhatRunsWhere allows you to stay on top of your competitors’ ad strategies. If something changes, you will know what they are trying and can see if it’s working effectively.
c. Get accurate data: You will get the latest data on your competitors’ advertising campaigns. It ensures that you have the most recent information at your fingertips so that you can make the best decision for your business.
d. If you find yourself asking, “where are my competitors advertising,” you can use WhatRunsWhere to find some answers to that question.
Cost: $299-$399+ per month
6. Moat: Moat allows you to look at different types of ad creatives like mobile, desktop, direct, and programmatic ads. You can monitor competitor ads to see different kinds of creatives a brand is running now and ad creative they have run in the past. If you use Moat as one of your competitor analysis tools, you will be able to:
a. Search for ads by brands: Curious to see what types of ads your competition runs? With Moat, you can search for any brand and get insight into ads your competition ran recently.
b. Visualize data: When you have data on competitor ads, you can visualize that information with Moat. Moat presents ad data with charts to help you understand your competitions ad performance.
c. See cross-platform performance: Moat allows you to see ads from different platforms and devices. You can see competitor ads from mobile, in apps, and on desktop.
If you want to learn more about where your competitors are advertising, Moat can help you find the data you need.
7. Adbeat: Adbeat allows you to monitor competitor ads and see what your competition is doing. You learn what type of ads their running and how they are running them. From the messaging to the images, you’ll get insight into the types of campaigns they run. With Adbeat, you will be able to:

a. Gain valuable insights to inspire your ad campaigns: Adbeat allows you to see information about media buying strategies, landing pages, and more from your competition. You will have valuable data that helps you understand how your competitors run ads successfully.
b. Learn about different ad networks: When you use Adbeat, you will learn how competitors buy traffic from various ad networks. It will help you see if there are potential ad networks for your business to drive traffic.
c. Learn about your competition’s buying strategy: Want to know how your competition goes about buying ad space? With Adbeat, you get an in-depth look into your competition’s publishers, ad types, and more. This information allows you to find new traffic sources and placements so you can grow your business.
If you want to monitor competitor ads to help you create a more effective ad strategy, Adbeat can help you learn more about your competition.

Now to answer the second part of your question the most cost-effective Ad tool are as follows:
1) Keyword Research Tools
a) 1. SEMrush
b) 2. SpyFu
c) 3. Google Keyword Planner
d) 4. Ahrefs
2) Ad Creation Tools
a) 1. Canva
b) 2. Smart Ads Creator
c) 3. Crello
3) Landing Page Builders
a) 1. Instapage
b) 2. Unbounce
c) 3. HubSpot Landing Page Builder
d) 4. Leadpages
4) Reporting Automation Tools
a) 1. Optmyzr
b) 2. Reporting Ninja
c) 3. Marin
d) 4. Supermetrics
e) 5. Adstage
f) 6. Google Analytics

Besides if you do have any questions give me a call: https://clarity.fm/joy-brotonath