Amtrak to Oakland and BART to SFO.Parking at SFO long term lot will still run you $10 a day or so.
Get a friend to give you a ride to the Amtrak station. Take Amtrak to the Richmond station. Once there you jump on BART to SFO.Amtrak costs about $25.00 each way. BART is another $10.00Ignore the answer that suggest you go to Oakland. You are overshooting your goal.When you consider gas, parking, bridge tolls this is a way cheaper option.
take the metro or the train