It's all open source and free, on Github. I've used .NET for about 10 years now, and the only money it has cost me is a few third party libraries that I couldn't find good open source competitors for.
Microsoft owns the .NET trademark but the language and source code for the current runtime is freely forkable and usable by any person/company who wants to do their own implementation. It's no different than the situation with Oracle and Java.