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Pay per play: exploring the pros and cons of freemium gaming on iOS.
For any number of reasons, "Pay per play" has seemingly become the de facto method for many game developers to produce their titles.
There are pros and cons for both the developer and the customer and Craig Gannell has a very good overview of the issues from many perspectives. http://reverttosaved.com/2013/04/26/pay-per-play-exploring-the-pros-and-cons-of-freemium-gaming-on-ios/