Wirelessly posted (BlackBerry8530/5.0.0.601 Profile/MIDP-2.1 Configuration/CLDC-1.1 VendorID/417)
Suggest that you price the rental initially at an online TA like Expedia or Travelocity, which will display rates from multiple companies. If you have any discount codes or applicable coupons, you can then book directly at the website of the car-rental company you choose.
Also, if it's an option for you, don't overlook the possibility of picking up the car at an off-airport location -- like Jersey City or Metropark -- which might be cheaper.