A little inside info on last month's test case. Our own bartmoss is running a site called Enderra. We put up a banner for Enderra with a measly 10000 impressions on the ForumPageTop and ForumPageBottom ad units which would be $3.50 CPM. This was a 1 month campaign that would cost $35. Out of those 10000 impressions he got 132 clicks. This boils down to approx $.26 cost per click, whereas our adsense average was $.31...a savings over using adsense to bid on clicks. Breaking it down further to the individual ad unit the ForumPageTop banner showed 4311 times and was clicked 99 times. Had this been run individually for $5 CPM, this would have been a $21.56 campaign with a cost-per-click of $.22. The ForumPageBottom was shown 5,694 times, and clicked 33 times. Had this been run individually at $2 CPM, it would have cost $11.39 for 1 month with a cost-per-click of $.34. Not quite as high a performance, but clicks are going to be a bit harder to squeeze out of below the fold banners. Bartmoss also tells me that his banner didn't really get a lot of design consideration, so if a better banner were designed, it would no doubt have gathered higher CTR's.
Overall, aside from those 132 clicks, he ended up increasing his monthly pageviews by much more than the 132 clicks because of return visitors and having the occasional banner ad to maintain an identity.
I hope this helps explain things a little. The advertisers packet attached above is going to be a "living document" that grows with the site's statistics, so expect it to change over time. I'm going to be working on getting much more information in there regarding the ad placements and units as well as some information about what kind of banners you can post (animated and flash banners ARE allowed if they're not ridiculous), as well as information about how you can track your ad's progress through a campaign (you get a customer login to our ad system with access to reports). If anyone has any questions, feel free to email me, post them here, or PM me.
P.S. I'm also considering creating a sponsor group that will add a title to your username when you post. We'd love to have you as our sponsors. This money will go towards getting us moved over to dedicated web hosting. Let me know how you feel about this.