With what prices, what items should maker give to users?
Of course this depends on specific game, but there can be good strategy about setting price structure, etc.
There is a flurry blog article from 2011
http://www.flurry.com/bid/67748/Consumers-Spend-Average-of-14-per-Transaction-in-iOS-and-Android-Freemium-Games#.U3lgoq1dVvY