I've taken several salary surveys and done plenty of research through publicly available sites (e.g. glassdoor, linked in, payscale, etc.). For what it's worth, my all-in compensation is well below $300k/year as a senior engineer and is typically reported as being on or above the very high end of average. Given the propensity to exaggerate in these surveys I expect my salary to be even farther from the average than reported.
Definitely agree with respect to opportunities, though. Even at double the cost of living or slightly more, the doubled salary means one is saving more in absolute terms and has access to a much better job market.
I got my current job applying through the website knowing nobody at the company. There are just more (and better) opportunities here.