We just returned yesterday from a great 8 day trip...
Parking at PHL is both insane and expensive... I took a tip from a Dis'er and used Winner Airport Parking. They are unique in the aspect that you drive to their facility, their driver takes you in YOUR CAR to your terminal, and then returns to the lot. Upon returning, you call them and they will bring YOUR CAR back to the airport to pick you up, at which time you can either return with them to their facility to pay your bill, or for an extra $11, you can leave right from the airport...
We chose to return to the facility to pay the bill. Using the coupon from their website we were able to get a decent discount. Our total for 8 days, with taxes and surcharges, came out to $66. We also tipped the driver $5 each way. I think this is probably the most affordable and convenient service I've found so far... and I'll definitally be using them in the future...
Hope this helps... winnerairportparking.com