Budget hotel parking from $9/day — free shuttle every 30-45 min
Travelodge SFO offers non-guest parking from ~$8.99-18.99/day. Has a separate Yelp listing under "Travelodge San Francisco Airport Parking" with 107 reviews. Also available on ParkSleepFly and ParkingAccess.
Free dedicated shuttle runs every 30-45 minutes. Note: some reviews report security concerns including car break-ins.

IHG hotel parking from $22/day — free shuttle to SFO & BART
$9-10/day, shared shuttle every 45 min — no midday service 11:45am-5pm
Park Before You Fly packages — hourly shuttle (4 AM - 11:30 PM)
From $13/day via aggregators — free shuttle every 30-45 min