Isn't it pretty rare to pay someone that much up front? I thought they all get back end deals, taking home the gross profits.
I'm assuming that this deal was signed the same time as the deal with Terrance Howard, which Marvel - then somewhat inexperienced as a studio and at signing such deals - later realised was unduly generous to the actor.
They tried to get Howard to agree to a reduced fee for IM2 but he understandably refused. When he did, he was replaced by Don Cheadle. Of course, RDJ is much more indispensable than Howard was, which is why they can't just replace him with Rob Lowe/Charlie Sheen/Emilio Estevez etc.