What is cheapest way to SFO from 18th street castro?

According to Google it would cost you $8.25 in gas to drive from 18th Street in the Castro to SFO (and then, of course, you would have to pay for parking which would be really expensive).

If you take the Metro to the Civic Center, you can take BART to SFO which, again according to Google directions, would run you $10.25. With luggage that might be a bit of a hassle on the Metro part but is probably the cheaper way if you intend on taking public transportation.

Hope that helps. Happy trails.