Housing can be very variable, to the degree there aren't any reasonable options in San Francisco even if you have a lot of people sharing a space. I don't think basic food varies nearly as much. A 5 pound bag of rice costs a little more in San Francisco, but not much more. I think there's less geographical variability in cost of living (excluding housing) if you constrain costs to basic needs.
Also a UBI shouldn't have to facilitate living _everywhere_. Santa Monica is lovely. Should we arrange it so anyone can move to Santa Monica and be able to live there fully supported on their UBI?
I hadn't really thought about transportation costs, and am inclined to leave it out of UBI calculations as it doesn't feel like a necessity. But in a rural area transportation is necessary just to buy food, so I guess that adds some complexity.