Actually in order from lowest to highest grossing:
Hulk, Cap, Thor, IM, IM2, Thor 2, IM3, Avengers.

There's obviously a pattern there, being that all the recent films have been their highest grossing ones. Marvel as a studio and brand has established itself by now, and folks are going for that more than for the properties that, let's be honest here, most people are far from familiar with.

The average movie goer isn't familiar with Thor, Black Widow, Nick Fury, Hawkeye etc. That didn't keep Marvel from turning films into hits. Generally they've spent about 150-200 million on each film, and 263 million is their lowest gross (Hulk). They're fine.