If you're enamoured with MVVM and know C# you might consider trying out Xamarin + MvvmCross [1]. I'm using it on a few projects and the code shared across platforms is (ballpark) 70-90% depending on the size of the project.
Xamarin's come out with Xamarin.Forms which looks like it could supersede MvvmCross but I gave it a shot and it's pretty immature right now. Plus its dependency injection leaves a little to be desired [2].
It might be an unfair comparison as MvX is a much older project but it felt more like something cobbled together over time than something really thought out. I also really like the reactive approach - your view model describes the relationships between its properties as they change rather than have a morass of interlinked event handlers.
Paul Betts is pretty active within the portable-C# community, with multiple active projects: https://github.com/paulcbetts
and has a pretty good blog, where at the moment he's going back through the RxUI docs: http://log.paulbetts.org/
Do I need still need to spend $17,091 a year to equip 3 developers with reasonably unrestricted Xamarin on 3 iOS, Android and Windows Phone? Or $8,991 if I don't care for hot fixes.
Xamarin's come out with Xamarin.Forms which looks like it could supersede MvvmCross but I gave it a shot and it's pretty immature right now. Plus its dependency injection leaves a little to be desired [2].
[1] http://www.codeproject.com/Articles/566270/
[2] Out-of-the-box it appears to have a global service locator and no constructor injection.