The Avengers made $1.5 billion US dollars worldwide. Assuming an average ticket price of $12.50, that's roughly 121.5 million people going to see the movie. I'll be modest and say half of those people are comic book fans who are diehard fans and would go to any Marvel or DC movie. Even then, when they eventually lose the other half of that fan base (and they absolutely will; to say they won't is ignorant and ignores essentially every trend in movie history), they will be left with ~$750 million in grosses on what is the most profitable venture they've had. Movie studios pay out half of their box office grosses to the theaters, which would leave $375 million to cover production, print and advertising costs. Of course, there is also merchandise and home media sales, a big reason why this trend is safer than most in Hollywood have been, but eventually these movies that have gone from niche to mainstream will fall back into niche and be taken over by nachos flying spaceships or whatever stupid trend will be big in a few years.