Good question. $10 buys the movie and as a "Thank You" you get a line of text on the billboard. $49 buys the movie + soundtrack and you get 1/4 screen text/image. $99 buys the movie, soundtrack, special features and you get full screen text/image and $149 gets an associate producer credit as well on top of this.
Basically, people are buying the movie in different packages and are getting the billboard message as a "Thank You" from us.