Critics are always saying that you can't make money from Android apps and that iOS is a better market for making money. But I think that depends on your app and understanding the two markets.
The Angry Birds franchise is a great example. On iOS they sell their apps to customers for 69p each. But on Android their apps are free on the Play Store. The reason for this is that in some countries customers can't buy Android apps, they can only download the free ones. Rovio, the devs behind Angry Birds, realised that. They also took into account that if they did sell the apps on the Play Store they would be pirated hugely. So instead on Android they implemented in-game ads. These ads run whenever you play the game. Also with them being there forever, Rovio keep making money every time a person opens an Angry Birds game. This has meant they make over $1 million a month from Android alone just from ads.
Sure there is a one a million chance of becoming as popular as Angry Birds but Android users don't buy apps as much and like their free apps. So one of the best ways to monetize on Android is to use in-game ads.