The Mechanic's Institute, near the corner of Post and Market, is a non-profit private library and chess club, and membership only costs $100 per year. That's considerably better than the $300 a month charged by the least expensive coworking spaces! It's also, in my humble opinion, a much more beautiful workspace. The building has tall ceilings and windows, large columns, wood-paneled study rooms, plush leather chairs, and large desks with power outlets. The wifi is also fast and reliable.
The reviews on the library's Yelp page will give you a good sense of what it's like:
http://www.yelp.com/biz/mechanics-institute-library-and-chess-room-san-francisco
I hope to see you here! It would be great to make this a well-known hacker outpost in downtown SF. Perhaps we could even organize a daily lunch or afternoon tea. Here's a chat room where we can coordinate:
http://www.tlk2me.com/chat/#mecha
You can also find my email in my HN profile. If you're visiting SF or want to spend a day here before committing to a membership, I can get you in as a guest.