Glad to hear your getting some good response from possible advertisers!
This is a tough question to answer without a lot more data. I'll give it a shot though:
I've seen banner "sponsorship" type ads on small to medium sized sites go for as little as $50 a month all the way to $2,000 a month. The factors to consider when setting your price:
- How many times in total will the banner be seen during the month? How many unique people will see it? (if only a few hundred people will see it, it's going to be very hard to get them to keep advertising after a month)
- How expensive is the market to advertise in online? (are AdWords ads in this market $1+ per click? Or are they $.05 a click?)
- What sized budgets do your prospective advertisers have?
- Do you want them to sign up for a multi month commitment? Or is it month to month?
- Is this an exclusive sponsorship where other advertisers will not appear on the same pages or the same placements within the page? Or are you rotating their banner with others?
- What size are the banners?
- What type of creative can they use? Can they place an html or interactive banners?
- Are you creating & designing the banner? Or are they?
That's a very general starting point to deciding your prices. An added tip: Sometimes when I'm going to price a new product or service I do a survey to figure out the price. I describe all the features and benefits of the product (the list above would be some of them for your ads). Then I either send traffic to a page or call people on the phone and actually survey on them on what prices they would consider would be: Too little, too high, almost too high & just right. That gives me a lot of data to decide my final price point.
I think the worst case scenario for you is that you'll price it too high & they'll say no. Then you'll have to call back some time later and "eat crow". just tell them you didn't mean to shock them with the price and would like to offer the same ads for 50% off the first placement just to prove to them how effective the ads will be.
Let us know how it goes. Good Luck!