This doesn't actually happen, except in the heads of naïve young dudes who want to justify living in SF right now.
Total REAL compensation for a non-senior engineer in SF is somewhere around $130k right now, with some Magic Stock Bux (probably worthless! maybe not!) thrown in for fun...but those don't buy burritos. Total comp of $110k sounds reasonable for Austin, as well. You can shift the ranges up or down based on seniority, but the spread is about the same, and the range caps out at around $200k in both markets for all but the most exceptional people.
In other words, the salary premium they're talking about here sounds right to me, and a premium of $20k a year is more than consumed by the 3x difference in rent, alone. You can perhaps justify going to SF on other grounds (career growth, networking, etc.), but it's not purely economically rational for most people.