Rapid Rover is the cheapest around in my experience for PHL:

http://www.rapidrover.com/

They run a shuttle van service that picks up multiple passengers for the trip. Depending on how close to PHL you are you may have a quick ride or a long one. Leaving the airport is usually about a 20 minute wait for pick up. Or at least that's what it was when I stopped using them a few years ago. I'm not sure if they do EWR.

I use a small company called Safecar. It's more expensive, but where I live I was always the last off Rapid Rover, so driving all over South Jersey dropping off the other customers after my flight from the UK was not my idea of a good time. They do go to EWR, but the fee is triple digit. Sorry, I don't have the number to hand to post it, but if you PM me I will get it for you.