Lots of threads on SF hotel recommendations and travel tips. Skim back through the threads in this forum. Unless a huge conference is in town, I would think you would have lots of choices of good hotels within your budget. www.quikbook.com is often a good place to look. For a less well-known chain, check out http://www.jdvhospitality.com. I have not stayed in their hotels in SF itself, but I have stayed at others in the area and had generally excellent experiences.

(Disregard the following as I was confusing Clift with Cliff): Clift Hotel is on the far western part of the city, and can be a long bus ride to typical places where first timers want to go. The broad area around the intersection of California and Powell would be a much more central location.