DFP for Small Business is what was formerly known as Google Ad Manager. The first 90MM non-AdSense monthly impressions are free, and then you have to pay. To clarify, say you have 200 MM monthly pageviews, and you serve 3 ad impressions per page. You would have 600 MM ad impressions. Say that Google serves AdSense ads to 2/3rds of these ad impressions, leaving you with 200 MM non-AdSense monthly impressions. You'd be over the 90 MM quota and you'd have to pay a hosting fee of around 2 cents per thousand impressions (impressions, not pageviews).

A newish self-service site to check out is http://www.isocket.com. They charge a fixed fee (and don't take a percent of revenues).