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Xamarin Starter Edition is free. There are some tough app size restrictions but apparently they want developers to at least be able to kick the tires!

https://xamarin.com/starter

Xamarin.

Xamarin is the company, Mono is the product, right?

Xamarin is the whole mobile environment (including framework), Mono is "just" an open source .NET implementation and a C# compiler.

then maybe kotlin is what you want ? It shares some nice features with C# and is entirely inter op with java code. Also, it is free.

The regular environment for Android is free. As is the regular environment for Windows Phone.