Monterey to SF, but cheaply?
 

Have you tried Google Maps, and clicked on the "public transportation" option? You can adjust the departure date and time; the default is the next available routing.

You can take a bus with a connection in Salinas or a bus connecting to Caltrain in Gilroy or San Jose. The total cost of the trip is anywhere from $14 to $29.

Take a look at Rome2Rio for details.

Have you looked at the airport bus from Monterey to SFO (and then BART up to SF)?

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

Anyone know if Uber Pool (or whatever they call their cheapest, share-with-others option) is available in Monterey? If so, that might be a great option. I once did an Uber Pool from my home in the Berkeley Hills to SFO for $21. Regular taxi fare for that would be over $100.

This one?

I am trying to find a better rate than that.

I'm nowhere near senior citizen-age, but it's a fair point. (You bring up a good suggestion, though...can one get the Clipper card at SFO - e.g. on a layover - without going to the BART station?)

Will check this out, too!

Take a look at Uber Pool, but it would be hard to beat TWA884's bus suggestion if you're going for the cheapest approach.

ETA: For fun, I just plugged in my home in Berkeley to the Monterey Bay Aquarium. No Pool option; regular Uber for $134. I'm guessing that the Pool concept doesn't work for such long trips between metro areas, with sparsely populated areas in between.

I believe you'd have to go outside of security to the either SFO BART station (which is attached to the International terminal departures level near the G gate side) or at one of the SFO Information kiosks in Terminals 1/2/3 on the arrivals levels. All these options are outside security though. If you have a long enough layover to re-clear security, you could probably do it.

According to the Clipper Card website page showing retail locations, you should be able to buy a regular adult Clipper Card at SFO at:

Terminals 1, 2, 3: Airport Information Booth on Arrivals Level 1 (in each domestic terminal)

International Terminal: Airport Information Booth on Departures Level 3, near BART station

https://www.clippercard.com/ClipperWeb/map.do

Sorry I didn't see this sooner, but for future readers, I am a regular between Monterey County and the Bay Area. Not as frequent these days, but still know it like the back of my hand.

The Monterey - Salinas Transit agency runs buses once or twice per day to/from San Jose Diridon or San Jose Airport, Bus 55 to/from Monterey and Bus 86 to/from Salinas. Fares are $12 adult / $6 disabled, senior or student. Day passes are the same cost, so be sure to get the Day Pass for any MST buses you may take before or after your long distance ride.

55 schedule to/from San Jose Diridon Station: https://mst.org/wp-content/media/55.pdf
86 schedule to/from San Jose Airport and San Jose Diridon Station: https://mst.org/wp-content/media/86.pdf

Another option is the Amtrak bus between Salinas and San Jose Diridon Station. By policy, you must book a train segment with any of the Amtrak buses. I usually book the multi-segment ticket as SNS-SJC bus + SJC-SCC (Santa Clara) train and throw away the train segment. Like airline tickets, book the train segment as the second segment so the entire ticket isn't cancelled for no-showing. This Amtrak journey is usually around $20-$25 one way.

Once you hop off either bus at Diridon, you can catch Caltrain up to San Francisco and points between, and vice versa for the reverse journey. Or catch the VTA Bus 181 to/from Fremont BART to get to/from the east side of the Bay.