For iOS I think you would have to figure out how to hook up MS runtime to a LLVM AOT compiler and then do all the work Mono team did to make sure libraries compile/work without JIT.
I really hope Xamarin guys follow Microsoft lead and make entry level version of their product free. It's not just about the ~25$ per month, it's about community as well - say some guy X is maintaining .NET library Y. I write up a port/pull for Xamarin - X goes "sweet but I can't test that since I don't own Xamarin and I can't even include it in my test suite to not break it further". Now I have to maintain a branch of Y for Xamarin and most other people are likely not going to find it or willing to use it.
I don't know which % of their profit comes from the indie licensing but if it's not significant as I suspect I would hope that they see this - it will help both their adoption and the OSS community build tools for their product.