If you commute into the city for work, look for cheaper parking lots on the fringes of Center City or even University City.
For instance, you can pay as little as $4.00 a day to park at Campus Park and Ride. It's on 1600 South Warfield Street in the Greys Ferry section of the city and the monthly rate for long-term parkers is $99.00.
That's much cheaper than a lot of the lots right around 30th Street Station. You can park your car in Campus Park and Ride's lot and then catch a free shuttle. The shuttles run 24-7. During peak hours, they run every five minutes.
Here are links for Park and Ride and other money-saving commuter lots and carpooling sites:
Campus Park and Ride
Ride Share
Phila. Up
Morey money-related links: