San Francisco was recently criminally insane in terms of housing costs. Now it's just criminally demented. (It's still criminally insane if anyone wants to actually buy something here.) Rents went down a lot following the dot com bust, but they've creeped up some since then.
A flat for three can cost you anywhere from the ridiculous ($3,500+ US/mo near the more desirable parts of town, like Nob Hill or Pac Heights) to the semi-"reasonable" ($1,800-$2,500 US/mo in parts the Richmond and Sunset districts -- where you'll occasionally encounter Indonesians -- and in some of the more Hispanic parts of town like the Excelsior and the Mission or East Bernal Heights districts, not far from where I am). Go across the Bay to Berkeley and north Oakland and you can save another 20-30% on rents.
After being gouged for rent, other costs are pretty reasonable by U.S. standards for food and other basics. In fact, produce here is cheaper than most places I've lived (Chicago, Maryland, North Carolina, etc.). Gas prices here are traditionally the highest in the country if you drive, but the local public transport can get you to a lot of places for $1.25.
This is an expensive place to be, but there's a reason for that -- it offers quite a bit. Especially for younger people.