A suggestion for Laurent Sansonetti will be to release this similarly with the way Xamarin has released Mono for iOS. Basically you should be able to download and use the library for free in the iPhone/iPad simulator and you will need to buy a license if you want to be able to export the app to the actual device. This licensing model has the advantage that is similar with what Apple does for developing apps for iOS.…
A question for the developers - did you consider this option and reject it for some reason?