I recommend reading "Will It Fly" by Pat Flynn and "The 4-Hour Work Week" by Tim Ferriss.

Go ahead and run a Google Adwords Campaign to a landing page with the option to purchase something related to your topic. If you get a good conversion rate of people who click the Buy button, then it is a winner.

Ideally, you are looking for a topic that is a huge need for your target audience. A friend of mine always says that he is looking for at least an 8 out of 10 on the BIG Problem scale.

The answer is highly dependent on you.

I run two highly profitable businesses + I'm just starting a third.

The keyphrase I targeted in biz #1 had zero traffic. I single handedly created the topic + related niche. Today there are 3M+ related links.

With biz #2 there's not even a front facing (public website). Last year's profit for this business was 2400%. All referrals come from talks I give at private masterminds.

With biz #3, search volume is 100/month (low) + first usage of this keyphrase was 1954 + the domain I own is an exact phrase match + holds SERP #1 for target keyphrase + many related keyphrases, with zero content on the site.

My approach is simple.

Never compete.

I take a keyphrase which can easily be dropped in conversation, which intrigues + then speak a few places about the top... okay, specific, highly target places to speak... which then creates traffic around a keyphrase.

This may seem like a lot of work.

If you choose a topic where you have high expertise + interest, which you talk/write about daily, then this entire process becomes a zero work proposition.

Rather than work, you simply monetize your current areas of expertise.

In my experience, every topic can be profitable. The key is finding your niche and target the right audience. The days of "If you build it, they will come." They've been replaced by, "If you build it the way they want it, they will come."

Most of all? Do what you love.

Pamela is right. Any topic will work. The key is that you have to immerse yourself in that industry, so you should be passionate about it. You can't just depend on passively posting on the blog—the blog is just your entry point.

You will need to build relationships (online and offline) with others in your topic and eco-system. You have to become known in your industry. Then people will follow your blog, you can speak, etc.

Once you become involved in the industry, you will have opportunities to make money.

But, if you want to do it right, and go big, ignore making penny ante money through google (or anyone's ads) or sponsored posts. Focus on building credibility and making real links with people in your industry.