well Disney paid Depp whatever he wanted because he was really the only draw they needed with that franchise, pretty easy decision there.
RDJ is still the main draw though. Sure, the others are relatively important, but Iron Man is not only the "leader" in the movie, but also apparently RDJ is their "leader" IRL as well. They're all stars, but he's the most bankable one and the main draw.
This situation is pretty unprecedented
Trying to think of a similar one...the Oceans movies maybe? Ensemble cast, but Pitt/Clooney were the main draws.
Also, with Pirates, you had Keira Knightley, Orlando Bloom, Geoffrey Rush, etc. While Depp was the obvious main draw, there was some other star power as well.