I seem to remember that they use to be around $50 to $55 per person.
We usually rent two one-way rentals from AVIS for less than $50 each way (plus gas and tolls). This way we are on our own schedule and not Disney's. After the cruise we took the car to Vero Beach to check it out.
The AVIS location in Port Canaveral is about 1 mile from the terminal and they offer complimentary transportation from AVIS to Disney Terminal.
We have also used tiffany towncar. This was about $200 for the roundtrip.