The more you think about salary based on location the less it makes sense. I got an offer for FB London that is as half of offer for bay area. Where does that $

The more you think about salary based on location the less it makes sense. I got an offer for FB London that is as half of offer for bay area. Where does that $$$ go? Maybe some tax, but then a bay area office is also more expensive. I'd be doing the exact same work. Coming from a freelance perspective, the only equation should be 'how much value do I provide company X, how much of that do they give back'