Martin would set the price for his app. Then Apple would get 30% of the sales revenue, and he would get 70%. If the app is free, then Martin wouldn't get anything because as Leo said, 70% of nothing is still nothing. It does mean, however, that Apple also gets nothing despite the services they are providing such as bandwidth and promotion of the app. Both Martin and Apple have a vested interest in a rich app environment and free apps are part of that.