In any airport I frequent, this would not be worth the money. Some garages offer free parking for 30 minutes for pick-ups, but by the time you park, go through security, eat, and return you would owe at least $4. Then there's the time/gas cost of getting to/from the airport. Even if you used public transport it could be expensive, it's like $16 round-trip on BART from SFO to downtown San Francisco.