There are a few factors:
- You have to add marketing to the production costs. A modest estimate is in the ballpark of $350 million total.
- The studio rakes in about half the box office. So, it'd need to make something between $500 and $700 million to turn in a profit.
- When a studio smells a bomb, they funnel all losses from other movies into that one, and write it off as a single failure for their investors.
Also note that Disney announced the "flop" after only 11 days. John Carter has been the #1 movie worldwide for two weeks, and it hasn't even opened in Japan yet. It is the largest opening ever in Russia. It just opened in China, raking in great numbers.
Also telling is how Disney decided, about six months ago, not to license anything related to John Carter. No Lego, no toys, no action figures, backpacks, clothing, videogames, nada. That was one of the movie's hopes of turning in a profit. But everyone at Disney who greenlighted the movie is no longer there, so the movie had no one to champion it.