so as you know $100 won't get you very far, especially for a fixed gear. you're better off spending that $100 on a regular road bike that fits you and saving up for a fixed gear. That way you know what you want and have a bike while you are putting it together.
how tall are you? or what is your inseam?
here's one in pacifica
http://sfbay.craigslist.org/pen/bik/714226918.htmlThis one is not working but it's only $60 so you could spend the other $40 fixing it
http://sfbay.craigslist.org/eby/bik/713864823.htmlGet this one and I'll buy the panniers off you! this one is the closest
http://sfbay.craigslist.org/sby/bik/713543775.htmlthis one is far and you'd probably have to be tall
http://sfbay.craigslist.org/nby/bik/712610213.htmlone in santa cruz but needs a little work.
http://sfbay.craigslist.org/sby/bik/712572329.htmlthere are more I just need to know what size you ride.
hit up yard sales on the weekends, you can often times find a good bike and dont forget about buying lights, a helmet, and a lock. you can get a helmet at the good will for cheap and I know they have half way decent one's