Originally Posted by PatTheSlat

From what I've heard SF usually has way higher bike prices than pretty much anywhere else, but after a quick look at craigslist it seems there are a pretty good number of mountain bikes already set up with slick or semi-slick tires (which are what you will want for road-use) for $100-150.

Depending on how tall you are...

5'4-5'5ish: http://sfbay.craigslist.org/pen/bik/4340799661.html
5'7ish: http://sfbay.craigslist.org/sfc/bik/4344090426.html
5'9ish: http://sfbay.craigslist.org/nby/bik/4340781249.html
6'+: http://sfbay.craigslist.org/eby/bik/4344015488.html
Oddly enough all of those are Specialized, there's nothing particular about that brand those just happened to be the first few decent ones I saw.

If you tell us how tall you are we can find some more for you.