He is reportedly getting as much as $50 Million while Chris Hemsworth (Thor), Jeremy Renner (Hawkeye), Chris Evans (Captain America) and Mark Ruffalo (The Hulk) will be paid somewhere between $2M and $3M.  Meanwhile ScarJo (The Black Widow) and Samuel L. Jackson (Nick Fury) will get $4-$5M. 

Nope, it's not about favoritism. It's just because the success of the Iron Man movies led to Downey's contract being upgraded which gives him a percentage of the earnings of every Marvel movie he's a part of. Smart!