Is this a round trip plan? If so, it seems like you'd need to double up your mileage
For one leg at 1100 miles consider your gas consumption ... if you get 25 miles per gallon you'll need about 44 gallons of gas to make the trip .... at about $3.50 per gallon you are looking at over $150 in gas each way ...... so you need to plan for $300-$350 in gas round trip if your mileage is in that range (if it's lower obviously your costs go up .... at 15 miles per gallon you'll need around 73 gallons or $255 each way) .... also the obvious thing to try and guess is gas prices
aside from that - what are you looking to do in philadelphia? are you staying at a hotel downtown, with friends/family, etc
if you are in center city you're going to have to pay for parking at the hotel - if you drive to some attractions there will be parking to pay for there as well ....... public transportation is pretty good in the city and there are a lot of walkable areas - you could take the train/cab from the airport to center city and then get around on bus/train/cab/foot to many of the attractions and then use something like the philadelphia car share if you are venturing out a little
Philadelphia Car Sharing with PhillyCarShare - Philly Non Profit Car Share Company
basically a lot of variables - but in some instances having a rental (or a car at all) for your visit in philadelphia is not needed which can cut expenses
you also have to consider that RT you'll be spending a solid 3 days in the car ... if the vacation time is fixed it's less time at your destination - but it could provide opportunities to see stuff along the way .... that will depend on what you are looking to get out of the trip
a ton of variables to consider