I'm pretty sure you're calculation is right Verge (although you could check it on any web mining calculator). The main reason for this is people hope that if they sell off their GHs, the mining profits and the sale will give them a net profit and hence from a pure mining standpoint you'll always lose money by mining there. It really just is a trading platform using GHs as a commodity which happens to have the ability to mine while constantly devaluating as a result of difficulty increases.