I'd say it's not so-much the cost of living which really drives the pay ranges, it's more the level of local competition for talent. The actual cost of living difference between, e.g. metro-west Boston and Bay Area probably can explain less than half of the salary difference.

When US devs talk about their wage it always seems absurdly high to me as someone from the north-west of the UK, but I know you have to factor in health insurance and living costs (particularly if you live in the Valley or Bay areas).

Even so, after factoring in living costs and all the other differences, they're probably still making two or three times what I am, with less experience.