That's true, I chose the advertising method for this app, although its probably slower than selling a paid app on the App Store or Android Market. The biggest factor that led me to decide on a free app was the ability to gather some useful analytics. Publishing a paid app might make more money in the short term, but only get a total of a few dozen installs. Since this was my first app, I thought it would help to do some market research first. Now there are about 1000 active users which gives me a good indication of where to focus future development effort. :)