NJ Transit 'off peak r/t' fare from Trenton to NY Penn is $15.75--not sure what the SEPTA fare is, but can't imagine it's too high. IMO take NJ Transit, rather than Amtrak. It runs on the same line all the way up to NYC and is much cheaper--sure its a bit longer (as it makes more stops), unless you are in a big hurry. If you need it, send me your address and I'll send out a schedule, although if you're not travelling until Labor Day, I'm sure there will be some changes to it before then. Additionally, if you were to do the Chinatown bus, you'd have to get into CenterCity from the airport first, this way you can go directly from PHL to SEPTA to NJ Transit (the NJ Transit ride takes about 90 min).