I think he did the first Iron Man for very little money, so considering the fact that he was an integral factor to the success of that movie, he deserves a big pay check now. But honestly all the actors did a great job in Avengers. Hiddleston, Ruffalo, Evans and Hemsworth. Hope they all get a good portion of the profit.
I think Robert Downey Jr has done amazing thing for MARVEL, his portrayal of Stark/Iron Man has brough that dark side/humor to his role and without him it just wouldnt be the same. TOTALLY PRICELESS!!