I have a monthly azure credit of $150 and some reduced pricing simply by having a ms developer subscription. I'm guessing Microsoft MVP's (in general, and Azure MVPs perhaps in particular) have extremely generous azure credits so hopefully he isn't on the line for the full amount here.
Xamarin isn't based on the .NET Framework. It is based on Mono which is in turn already consuming a lot of code from CoreFX (== .NET Core) and that share will only increase over time.
Source: I'm working on the Mono BCL+Tools team at Xamarin/Microsoft.
It's true that .NET Standard 1.0-1.6 had a lot of gaps but these should be rectified with 2.0 now so you shouldn't run into these problems anymore (otherwise there's usually a good reason why a method/API is missing from the standard).
Disclaimer: I'm working on Mono for Microsoft/Xamarin, primarily class libraries and tools.