NEW
This library has been moved to:
https://github.com/xamarin/XamarinComponents/tree/master/Android/GoogleAndroidVending
It is also no longer a C# port, but the official build from the SDK manager.
These docs are a bit old, but i cover everything:
ORIGINAL
I have created the C# version of the licensing library. I am currently using it in one of my apps on Play.
Check out my repo:
Android.Play.ExpansionLibrary
It has a couple of samples, including a simple licensing sample and a expansion file sample. Let me know how it goes.
I know that a guy at Xamarin is working on a tutorial now, but maybe you could put a request for it sooner.