MonoDevelop is free, but if you have a large code base, it's very slow. As in "wait 30 minutes for MonoDevelop to start and analyze the entire project" slow. And then the crashes...

Xamarin fixes all of these problems. But then, the free version is limited and the paid version is very expensive. When it comes to IDEs, you really do get what you pay for.

In my opinion a developer should always pursuit the better setup to be able to use all the features that a platform offers. Visual Studio Express lacks important things that the Ultimate Version has.

For example, plugins support.

You only need the pro version for plugins.. Although I haven't used express so I cannot speak on if it supports plugins too..

With the professional you lose test coverage =/ but yes, you have plugin support.

There is also SharpDevelop too.

http://www.icsharpcode.net/opensource/sd/