It's no use talking about Hollywood budgets and "breaking even". These are the same types of folks who claimed the "Lord of the Rings" movie trilogy didn't make a profit which prompted Peter Jackson and New Line to get in legal tassles over. New Line ultimately paid Jackson big money to settle the issue.
I'm sure there are some folks in Hollywood that would have you believe James Cameron's "Avatar" still hasn't turned a profit.
Most "informed" ie; not braindead folks say that in order for a movie to "break even" it must earn back its production budget plus another 40-50% on top of that for advertising and print duplication, etc. I tend to think that any movie doubling its budget is successful.
"Prometheus" with it's worldwide total has already come close to breaking even and will end up with a nice profit for Twentieth Century Fox. Add in the eventual 3-D Blu-Ray, Blu-Ray, DVD and digital download copies when it goes on sale for the home market and it's looking even better.
Day 1 for the 3-D Blu-Ray release for me btw.
I suspect we'll hear Fox making an official sequel announcement before the end of the year.