I also live is SF. If you want a cheap bike, you must buy second-hand. My wife bought mine off a friend for $200 (discounted because of the friendship, but still: it happened.) A coworker of mine scored a bike for $40 after a conversation with someone that was riding a bike they didn't like. It's possible to find cheap bikes, you just have to be willing to buy something that may need some work.

The attitude of many cyclists in this city needs improvement, but there are lots of organizations in the city that do their best to help out new cyclists, like the Bicycle Coalition and the Bike Kitchen, and on Bike to Work Day you can find a lot of really helpful people.