This is anecdotal based on my experience in both the Bay Area and SLC, but the pay differential is extreme at the top end, and practically nonexistent in the middle bands. That is to say, the kind of developer who might be able to command a $400K+ salary in the Bay Area may be able to get $200K in Utah. But if you qualify for the kind of position that pays $150-180K in the Bay Area, maybe at a startup with equity, you'd get about the same in Utah, maybe a little less. But you can buy a really nice house in Utah for $500K.
That rings true to me - mostly just due to the companies present. I was near $400k in CA, and now around $200k here in Utah. But frankly, the lifestyle you can afford here on two >$100k salaries is ridiculous.