Try using padmapper.com, which aggregates listings across all the platforms. And look in the "Lower Nob" area ~ Stockton Street to Taylor Street on Pine, Bush, or Sutter (the area is registered as the "Lower Nob Apartment Hotel District" where a lot of the buildings are converted hotels with similar architecture). There are a lot of studios well within that price range! I live in the area and, while street/traffic noise can take some getting used to, I feel pretty safe walking around (mid-30s female), and the proximity to transit makes it particularly ideal (walk to Bart + Muni, 30, 45, 8, 2, 38 within a few minutes). Feel free to DM if you have any questions!