the eastern part of the Mission district/Bernal heights/ potrero hill area. Kind of out of the way of mainstream SF living, so its cheap, but its right in the middle of where a lot is going on right now skatewise- Potrero Park, the mission skateshop, and Pop's are all right nearby. Bernal and Potrero have some sick hill shit.
No offense to twitch or anybody else living in the area, but I would recommend against the sunset and richmond, kind of far out there, constantly foggy and shitty. Its cheap, but its kind of disconnected from a lot of the city.
Oh, and I can understand the drive to live in SF proper, but a good chunk of living there outside of skating is a huge pain in the ass. I moved to SF despite it because I didn't grow up dreaming of moving to the bay, I dreamed of SF. But recently I moved to West Oakland, and although its not the safest place in the world, its so much less of a hassle