The best way to determine the value of your advertising space is to look at the cost of advertising on other sites similar to yours. Many sites that accept advertising have this information posted in a special advertisers' section of their website or it is available in their media kit. Take a look at your top 3-5 competitors and average out their advertising offerings. Start with what the market will bear and if your website offers a truly unique advertising opportunity, you might be able to increase those numbers to increase your advertising revenue.

Good luck,
Lena