I think several people are confusing the SFO Marriott in Burlingame (near the airport) with the downtown San Francisco Marriott near the Moscone Center.
I may be wrong, but I thought the submitter was asking about the Burlingame (SFO) and San Mateo properties.
Parking at the SFO Marriott in Burlingame was around $13 for self-parking and $18 for valet the last time I was there. I suppose it may have gone up, but $24 sounds about right for the downtown property.
-David