How can I get from Newark Airport to Philadelphia for cheap?

Amtrak. Take the AirTrain to the Amtrak stop at the airport.

I was going to recommend Dave's Limo service, which is well known in Phila. They take people door to door to and from all the airports. I believe it's about $45. I haven't heard anything bad about them. I have used them way in the past, and didn't like it, because they make so many stops. But I just went into their website. I read some of the reviews. Some were horrible. Others were great. Their local phone number is 215-288-1000. Their 800 number is 1-800-255-2378. At the ground transportation desk, you dial 23.

You can take Amtrak but that's at least $50. Easiest is probably NJ Transit as somebody else suggested. You take it to Trenton, then switch to the SEPTA (Philly local rail) at Trenton, the R7 line. I think that they'll be able to sell you both tickets at once in Newark. Go to the NJ Transit website http://www.njtransit.com and type Newark Airport and Philadelphia into the trip planner, they'll give you all the details. It's probably about 2 hours altogether, and costs about $15. I believe it's pretty easy to get from Newark airport to the Airport Rail Terminal, but I don't know if you have to pay for a shuttle (the way you do at JFK for instance).

the turnpike i think i-95