I made the money back heating my house in the bay area in 5 months. The Nest is just much better at using energy efficiently.
I live in Wisconsin where I would be hard pressed to spend $300/month during the worst imaginable weather (and this is five months... the worst imaginable weather is .. January. Three months earlier or later isn't too bad, back to $100/mo or less). With the recent polar vortex event settled right over my house, maybe this will be a $400 month for me... maybe not.
I assume by "bay area" and spending $600/mo on heating you mean Prudhoe Bay on the north slope of Alaska?
a) It is a historically cold year for a prolonged period of time
b) You set your heat to 80F all the time
With gas heating, it looks almost impossible to save $250... unless you're heating an NBA arena.