Makes sense as the email inquiry I sent has come back saying practically the same thing:

Our storage rates are based on the size (not weight) of each piece. Most pieces are around $12-$25 each for each 24 hour period. (Our minimum charge is for 24 hours.) We can take just about any size piece in our secure storeroom. We are open every day from 7AM to 11PM, and we are located on the departure level of the International Terminal, on the "G" side, outside of the security area, so we are accessible to all travelers. We are just down the hall from BART. You can find maps at www.flysfo.com. We take cash, traveler's checks, Visa, MasterCard, and American Express. You do not need to make a reservation, and we always have room.

If I store a small carry on for $12-$15 for 9 hours isn't too steep I guess so it is what it is.