It's no secret that Hollywood actors make buckets of money for nearly every project that they do. The bigger the movie, the bigger the paycheck. This is even more true for massive franchises like the MCU.
Marvel movies have some of the most passionate fans in the world, meaning people flood the theaters for every single one. For the Avengers films, the actors and actresses were paid millions to play beloved characters. Robert Downey Jr. played the role of Iron Man in the Avengers films, and he was handsomely rewarded.