Hosting sites can be paid via CPM pricing, click-through pricing, or outcome based pricing.
Amazon.com pays for an ad banner when it results in the sale of a book.
Novak, Thomas and Donna L. Hoffman (1997), "New Metrics for a New Media," Draft of Paper appearing in World Wide Web Journal, 2 (Winter), 213-45.
Nielsen, Jakob (1997), "Do Websites Have Increasing Returns?" Alertbox April 1997.