The Richmond and Sunset districts are less expensive than many others, but still have easy access to transportation, great shops and restaurants, and are near Golden Gate Park. Other neighborhoods further south in the city (excelsior, parkside, etc) tend to be even less expensive.
That being said, the city is an expensive place to live in general, and if you're looking for something less expensive, or if you want more space for your money, you might want to check out some areas just outside of the city like Alameda or Oakland.
I'd be happy to help you on your house hunt if you'd like. Don't hesitate to contact me anytime!