Seconding SlimRiders recommendation to join your local bike coop.

Looking at sf craigslist, a couple possiblities (fit is critical and you haven't given a size.:

http://sfbay.craigslist.org/eby/bik/3067119987.html
A 1999 Trek 800 for $140. This is a mountain bike, but with slicks and cruiser bars would be a very nice commuter, and fine for longer rides up to 50 miles or so. This one will be a bit heavy.

http://sfbay.craigslist.org/eby/bik/3041805706.html
Univega road bike--I'd figure on spending $100 and putting indexed bar-end or thumbie shifters on it.