As major cities go, San Francisco is fairly expensive. As Calam has already said, eating out can be a very major expense. But if you're not looking for luxury in your sleeping quarters, there are any number of small hotels with quite reasonable weekly rates; many of them include a small refigerator and microwave as part of the room equipment. Coffee maker with daily supply of coffee is de rigeur
virtually everywhere. My own feeling has always been that I'm not actually going to be living
at my hotel. During the day I'll be out sightseeing. At night all I'm looking for is a comfortable bed. So whether or not there are nice pictures on the walls and whether the wallpaper matches the rug are not matters of major concern for me.