[mnglkhn2]

When unity got their mono license xamarin wasn't yet a business, hence the license was most likely zero.

[shadowmint]

It would have been zero to Xamarin, but doubt it was free.

I imagine they bought a perpetual license to distribute sans-GPL the full code of a specific version of mono from Novell.

This thread has a bunch of speculation and a few comments from unity developers: http://forum.unity3d.com/threads/new-version-of-mono-with-un...