Good suggestions already given.
However, if you're looking for a slightly cheaper alternative I would try the indoor parking lot at Thomas Jefferson University. I used to live right around the corner from them, and they had the cheapest parking rates. Probably because they are technically a nonprofit or something. It's been a few years since I lived in that hood but you could park 24 hours for $10 in the late 1990s.
The garage is on 11th Street just south of Walnut. You'd have to walk a few blocks more than the other lots mentioned.
EDIT: Forget it ... I just checked Google Maps and it appears the place is under construction. I know Google Maps is not totally current but it's not worth saving a few bucks just to find out the place is closed for construction!
PS fogo rules