Actually, a friend of mine made $400 over a few weeks in a cholesterol study, and got free food too. Unfortunately, he wasn't allowed to eat anything else.
These things DO exist, but I sure as hell wouldn't pay $20 for a list, especially when most of these studies are over by the time any list would ever be published, not to mention the fact that it would be impossible to enter study that wasn't near you.
An easier thing to do would just be to visit the local university and check the bulletin boards. Usually they only pay something like $20 or something, for a few hours, or maybe $100 for more time, etc.