Xamarin offers a free starter edition[1] and you can request free full licenses for use with Open Source projects[2].

Getting started on your own project or doing an Open Source project that relies on Xamarin's tools is perfectly possible. In fact we use such free licenses to produce OS X and iOS versions of the Tomboy note taking app[3].

[1] https://store.xamarin.com [2] http://resources.xamarin.com/open-source-contributor.html [3] https://github.com/tomboy-notes