Can use the VS Express and Xamarin enviroment, to develop and sell

You can develop an app with Xamarin for commercial usage – there is no extra charge! We only require you to comply with Visual Studio’s licensing terms,

which means that in companies of less than 250 employees with less than $1million USD annual revenue, you may use Visual Studio completely free (including Xamarin) for up to 5 developers.

However after you pass those barriers, you would need a Visual Studio license (which includes Xamarin).

http://stackoverflow.com/a/37267465