With U.S. mobile phone companies, such an international data roaming plan can cost as little as an extra $10-20/month, depending on the amount of usage you pay for, and assuming you shut off the signal religiously when not using it. The GPS/mapping and public transit features have been invaluable to me in my last few international trips.
Even if you really really want to stay out in the sticks, like Ocean Beach, MUNI does serve these areas, albeit intermittently, either via MUNI light rail or via MUNI bus.
Again, keep in mind that MUNI is the City of San Francisco's own public transit system and is completely separate from BART, which is a regional subway. Also, per your earlier post, keep in mind that while SF cable cars are run by MUNI, there is a special $6 fare for those, while regular MUNI light rail streetcars charge $2 and offer free transfers to other light rail streetcars, and MUNI buses, for a 90-minute period. Also, once you get here, it will be worth it to get a Clipper card, which is a loadable card that you can use to pay for all forms of public transit, whether MUNI or BART.
Japanfan, when specifically are you going to be in SF?