[...] 15 Biggest Box Office Flops of 2011 (gointothestory.blcklst.com) [...]

"if Green Lantern was actually a flop, why is Warner Bros. working on a sequel?" Because probably it wasn't a flop. When you'll take into account tax credit (which is not accounted in the budget but in the financial plan), revenues from all markets (tv sales from free and pay TV worldwide mean a lot of money) and merchandise (lots of stuff for this kind of movie with high MGs) it's probably a success. And when you have dozens of digital characters, costumes and sets already designed in your hard disks, it will be stupid not to use them in more than a movie. The same will happen to John Carter of Mars, no matter if the income form theaters is stellar.