What is the average salary for an international student after an LLM in US and UK?

I think that in the US the average salary is around 11,000 US$ per month.

Ask UnderemployedLawyer (http://www.llm-guide.com/about/UnderemployedLawyer). He will give you the full picture.

11,000 per month is the average of all law firms or top law firms?

Yes of course. In fact, what I tell you is total average including not only monthly salaries but also annual bonus, etc.