Car trailer rental normally consists of two options:

1. Tow Dolly Trailer.  A two dolly allows the user to drive the front 2 wheels of the car on to the dolly and for those wheels to be tied down with straps.  Only the front 2 drive wheels are off the ground, while the rear two car tires remain on the road.  These rentals are usually less expensive, but can cause additional wear on the vehicle for long trips
2. Open Flat Trailer.  This is a utility trailer, normally with two axles, on to which the entire car can be driven on.  This is usually the best way to trailer a car.  These trailers are generally much heavier than tow dollies and need a larger tow vehicle to support the car and trailer weight.  Open trailers are also generally easier to load and unload without the need to tying down specialized tire straps.

Tow dollies rent for about $29.95USD per day while open flat trailers rent for about $49.95 per day.

Consider buying enclosed trailers from Millennium instead of renting.  On long haul trips you may be able to buy an enclosed trailer and sell it at your destination and save a rental fee.