I got this from
here I don't know if this covers it, but it looks like there is some pretty decent info on the site. (these paragraphs are about 3/4 the way down the page)
Pay-Per-Click Networks
Pay-per-click (PPC) advertising only charges you when someone clicks on your banner and comes to your site. However, this approach is subject to fraud by unscrupulous siteowners. One reliable PPC network is ValueClick, which has found ways to prevent fraud.
http://valueclick.com/ They charge advertisers 55 cents to 65 cents per click-through, and screen their network sites carefully to limit fraud. A minimum advertising buy is $5,000. The advantage of PPC is that you can better control advertising costs. If you know it costs 65 cents to get a visitor to your site, you can more easily predict the cost per sale.
Pay-Per-Sale (Affiliate) Networks
A pay-per-sale (PPS) network only costs the advertiser when a purchase is actually made. Another name for this is an affiliate program. Here the advertiser pays a set-up fee of $500 to $10,000. The affiliate program service provider has already aggregated affiliate sites that desire to link to certain types of advertisers. The advertiser pays only when a sale is made, typically from 5% to 15% of the sale price. The affiliate network receives 20% to 30% of the commission paid the affiliate, but in turn tracks everything carefully and makes all payments to the affiliates. The difficulty with affiliate programs is getting enough qualified affiliates to feature your site, since 95% of the affiliates signed up with the major programs have little or no traffic to their sites. The more expensive programs are BeFree
http://befree.com and LinkShare.
http://linkshare.com Lower priced programs are Commission Junction
http://www.cj.com/go.asp?69320 and Microsoft bCentral's ClickTrade
http://clicktrade.com I've been quite impressed with the aggressive way Commission Junction has moved to serve small businesses.
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