Chazza,
The GoNYC says it is metered fare from EWR to Manhattan, $45-$55. Then on top you add the toll for the tunnel, plus antytip (say 20%).
If a taxi driver tried to get you to pay more than that you refuse. If they persist you take their badge number and report them. The fare details, and number to report them are labelled prominantly at several points in the back of legitimate (licensed) taxis; the kind that you would get from the official taxi rank.
If you have any questions at the airport itself. you can ask for advice from the official despatcher at the taxi rank. They often also give you a slip of paper explaining the fare details, and your taxi badge id, AND ask where you are going and tell you what your fare will be.
I think it would be pretty hard to get ripped off in a licensed city cab these days.
HTH