Of course the other option, which would be the cheapest, would be to catch a New Jersey Transit train from the EWR station to Trenton, and then transfer to a SEPTA train to Philly. A one way fare will run about $17.00.
This is a much slower way to go (about 2.5 hours versus just a bit over an hour on Amtrak), the seats are less comfortable and not much storage space for oversized luggage, but you will save a substantial amount of money and the the NJT trains run quite frequently.