I don't know if you're a senior citizen or disabled, but if you are, a lot of the SF Bay Area public transit (like transit in many places) is discounted at roughly half off for seniors 65 and over and disabled people. It helps if you get a Clipper Card first, though. It's valid on 22 Bay Area transit agencies, though apparently NOT including MST (Monterey Salinas Transit); those areas aren't part of the 9-county Bay Area.
https://www.clippercard.com/ClipperWeb/index.do