I think Iron Man 3 was a very nice bookend to the current story. If Robert Downey Jr. becomes too expensive for more Iron Man films, I'd be happy just to see him in Avengers 2 and 3, and maybe pop up somewhere else.
If the $50 million deal is the correct figure being quoted (though who knows really), Disney should just fork out. It's nothing to them, and the Iron Man movies make it back within the first weekend anyway. Films with Downey in are big earners for Marvel, so he probably deserves it. Though has there been any firm confirmation that he actually wants to come back?