Another perspective:

San Francisco is more expensive than Boston. According to this source, San Francisco is 23% more expensive than Boston.
Both San Francisco and Boston are in the top 10 most educated cities in the US, so they have very educated residents.
For the most part, San Francisco’s weather is temperate and, within limits, predictable. Its summers are famously foggy, winters tend to be fairly mild. Boston, on the other hand, has more extreme weather: summers are hot and humid, and winters are cold.
California has such a wide array of beautiful sites to see, trails to hike, beaches, mountains — and all of them are so close that you can be down and back in a day. Food is great, and everything is so fresh due to California’s agricultural output.

The Vertiginous commentary again: SF (Bay Area) is apparently much more crowded than Boston and environs, thus more expensive to find housing. I personally know people who commute 2 hours each way in order to have a place they can afford while working in or near The City.