This is one area where I wish iOS and Android would copy from Windows Phone, apps can have a 'trial mode' (which often is used to just have a free, ad-supported

This is one area where I wish iOS and Android would copy from Windows Phone, apps can have a 'trial mode' (which often is used to just have a free, ad-supported version) that you can download without payment, it can be time-limited, launch-limited, a feature-limited (depending on how you choose to implement it, there's just a IsTrial() method to call and you can handle it however). If you buy the app it just starts returning false to calls to IsTrial() and the functionality gets enabled. Of course, there are plenty of apps that have 'free' or 'lite' versions in the Android or iOS marketplace, but they suffer from a critical flaw: data doesn't transfer between the 'free' app to the 'pro' or paid one because of sandboxing unless they implement some kind of cloud sync.