Programs: HH Diamond, Marriott Platinum, Hyatt Platinum, UA PoS
Uh, don't drive.
Seriously, parking in downtown SD will run you anywhere between $20 to $30+ per day. Clearly not as bad as SF or midtown Manhattan but, since it seems that you want to keep as much of your money as possible, parking is a poor way to spend it.
If you're really at a conference, find somewhere outside town that you can park your vehicle for much less per day, get a ride in (trolley, bus, taxi, etc.) and spend your time at your conference as well as walk around a whole bunch to explore SD.