Here's a practical article about ways for iOS and Android game developers to monetise their creations.
Free-to-play games are growing in popularity and quickly taking over the Top Grossing app spots on both the App Store and the Android Market. It makes a lot of sense, especially for social games that need to achieve critical mass for the best gameplay experience. The user adoption rate of a free product is 5 to 6 times higher than a product that costs even $0.01, and that viral spread is a great way to get users to refer you to the customer that will pay later. This option also drives strong category rankings because it drives more installs, which makes it especially valuable now that Apple has prohibited incentivized installs.
If you're thinking of going that way, have a read.