The cost of living difference between Minneapolis and San Francisco is HUGE. For the cost of an apartment in San Francisco you could have a huge house on 1/2 an acre of land in the Minneapolis area.
This calculator takes into account more, like housing, food etc: http://www.bestplaces.net/col/?salary=107798%20&city1=50...
"A salary of $107,798 in San Francisco, California could decrease to $56,716 in Minneapolis, Minnesota"
Which is much more in line with my personal experience having worked in both Silicon Valley and Minneapolis.
If you're a good engineer, it is not uncommon to pull down 107k or more in Minneapolis as well.