This is a great question that has come up many times over the years. Unfortunately, there is not really a great answer. If you are shipping full load freight like you commonly see on the interstate in those generic white trailers, then cost per mile is a common calculation.
However, if you are shipping a personally owned vehicle long distance it really comes down to a base cost to ship from one metro area to another. Once that a base cost to ship is determined then any additional charges are added in to calculate your total guaranteed cost to ship. Some of the common additions that are used in your car shipping calculation are size or type of vehicle, if your shipment is on route or off of the route of the major metro areas, the ratio of available car shipping spaces versus current car shipments on that specific route.
We would love to give an easy answer and a solid number for the cost per mile to ship a car. However, the cost to ship a car is just not calculated that way.