If it is about the money (and I wouldn't be surprised if it was), then I wouldn't blame Marvel. They are a business, and don't want to spend more than they think they have to to get a certain return on their investment. How much more money is bringing in Norton going to bring Marvel, in an ensemble picture? Certainly, replacing Downey would cost them, because people enjoy his performance as Stark so much. But the Hulk wasn't as memorable. Nor was Norton's performance.
Honestly, I wouldn't mind seeing Bana revise the role, if they aren't going with Norton.
And I should say that I'm a big Norton fan, and this news does bum me out, but I don't know that his being a part of the film would make a huge difference one way or the other. But I swear to God, if they put Adrian Brody in the role, I'm gonna throw something across the room.