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
origin - http://www.pipiscrew.com/?p=6316 can-use-the-vs-express-and-xamarin-enviroment-to-develop-and-sell