They mainly make their money off of provider ads however some (like exotics) make extra money by offering web hosting and page design services as well. Banner advertising also provides some additional income.-Glen

I don't know about the answer to your question, but I saw an interesting article on Slate.com about on-line escorts.  It says that Veronica pays LA Exotics $100 per month to list her. She says that the ad has produced more than 250 calls (whether that is per month or over a longer period isn't clear), out of which 3 new men have hired her (at $500 introductory fee plus $500 per hour).

I am advertised on a few adult directory sites. Not exotics, but I am on eros and pleasure guides. They get paid for everything from advertising to web hosting. They do all sorts of things. I figure they all work the same way.Sexy Shanna