Cory Doctorow [points to a BYU study](http://www.boingboing.net/2010/03/04/free-ebooks-correlat.html) that shows releasing a free ebook version may boost sales of the printed edition.
You’d love to see a bigger sample, and correlation does not imply causation. But to me, it suggests that increased sampling usually generates more sales than it costs.
Advance screenings of movies work the same way. When a studio expects good word of mouth, they are often willing to give up a day’s box office ((When you buy a ticket for a sneak preview of The Proposal, it’s actually counted towards another film, generally one from the same studio currently playing at that theater.)) in order to get more people talking about their movie. They’ll also conduct word-of-mouth screenings tailored to specific audiences. “Free” and “exclusive” are big motivators.
(thanks Howard Rodman)