Note that there are two SmartPark lots at PHL; for some reason I gnenerally end up at the Essington lot although the Scott Way lot is closer (and a litle easier to find).

Another lot I've frequently used is west of the aiport on Tinicum Island Road (Scott Way to a right at the USAirways hang.ar and then around to the west end of the runway)

Here is a Philly Inquirer article about parking. It's a few years old (it was published on 9.9.01) so take the prices with a grain of salt.