Let me start by saying that the answer I give is over simplified, you will need to expand on the ideas and principles. First let me say that to maximize profit you minimize cost and maximize revenue. This means that the cost you sell it for is not very dependent on the cost of the ad but more accurately dependent on the marginal cost. I am assuming your marginal cost is fairly constant. You set your price so that the marginal revenue is zero for a change in price. To give an example, if you have 50 customers paying $100 (50*100=5000) and you raise the price by $1 and you loose a customer then (101*49=4949) this means that $101 is to high, what of you lower the price $1 and you gain 2 customers (98*52= 5096) This was good. So you would keep lowering the price until you had no increase in revenue. If the cost of the ad (to you) stays the same then you don’t need to consider it in setting price, if it changes that you need to take it into account in a similar way. Google spreadsheet can actually solve this problem.

http://docs.google.com/support/bin/answer.py?hl=en&answer=139704

How do you actually find this price? Offer it at many prices, with discounts, rebates starting offers….. Yes these are marketing tools but also an opportunity to get data. The question you about those that think it is too expensive is one of the challenges of charging all customers the same price. Ideally you would charge each as mush as you can. You need to think of how you can allow some flexibility, size of the ad, color, fonts, strategies that would allow some to pay $200 and some to $50. This of course makes your revenue optimizing difficult more difficult.

There is a lot more that can be said about this. But if you think about marginal cost and revenue rather that total costs and revenue you are are your way to optimizing you business. Also apply this same strategy to your marketing.

Let me also add that as a startup or for other reasons you might place more value on the customer than just the revenue.