I have been trying to put some kind of advertisement deal together for various gaming related websites that have contacted me. My site is a forum that gets about 275,000 uniques / month and about 600,000 page views / month so its pretty big/legit... The only spot i have available (for any basic type of banner ad) is the footer (just below the whos online stats and forums) I do have adsence on my site and dont think replacing the footer adsence ads would like kill my CPM averages.. so this is not really about replacing what would be lost from adsence its about what is a fair price?
There are link price calculators on the web. Perhaps you can measure your link values there and take an avarage.
Try and go add your website to some place like Adbrite. Apply there as a publisher and if your site is as good as you say then you should get good deals on your website for letting interested advertisers put their ads on it.