Could anyone please suggest an easy and cheap way to get to San Francisco from Monterey.
.....probably the easiest and cheapest would be to take the airbus from monterey to sf airport and then bart into the city from there.
You can get Cal-Train. The MST (local bus in Monterey county) has a connection to Gilroy. Then take Caltrain shuttle to Caltrain. It does not run often so check.
Most rental car companies including Hertz and Budget do not have dropoff charges for one-way rentals from Monterey to San Francisco. So depending on how many are traveling, that may be a good deal. I think it will run about $60 for a 24 hour rental.
It's also quicker and easier than some of the other suggestions because you can rent a car at the Monterey airport and return it at several locations in the city or at San Farncisco airport.
We sometimes do that to get to the airport and avoid steep airport parking charges while we are gone on longer trips.