Cost of living anywhere in the Bay Area is much higher than the national average due almost entirely to real estate.
Median household in SJ earns ~120k. Median for USA is ~$60k.
Even if the SJ resident spend 40% on housing, they come out WAY ahead of the median household spending 25% on housing.
Average cost of a house in America is around $380k, average cost of a house in San Francisco is $1.7M according to Zillow. San Jose is $1.2M. Fremont is $1.1M. The cheapest (and not a great place to live and horrible commute) place in the Bay Area is Vallejo and that's still $450k.
So 3-6x more expensive depending on where you look generally.