Comments like this have been made on the Xamarin forums a number of time - that it's too expensive for 'hobby' developers. I'll make the same comment here as I made there, that as a Xamarin Mac (Indie) developer I'm happy they are NOT aiming their product at hobby / fun developers. A hobby is something you do optionally, if you have the funds or time. If you can't afford it, or can't monetise it - which I think is still an option for 'hobby' developers - then you can't do it. It's the same if the hobby is skydiving, parachuting etc.
As regards lack of open source material, it's not important to me however as Xamarin is C# based then any OS project that utilises C# potentially can be used within the Xamarin ecosystem.
There is a significant 'use-case' outside of the Hacker News / open source community that Xamarin addresses. Just because it doesn't fall within the parameters set down by (I am guessing) the majority of posters here, it doesn't mean that it isn't a good or useful product.