Whether to and what to charge may depend upon how far they are going with it.
I couldn't find the Montana freeware.
Cost per mile depends upon how many miles are driven each year. The more miles, the less cost per mile for fixed costs.
I think what you're after is to recoup the variable costs such as, but not limited to: Tires, Oil changes, transmission fluid changes, general maintenance or estimated maintenance, brakes, etc. Fuel as well, if your rig gets 10mpg average and they didn't add fuel, you could use the mpg to estimate.
You could also throw in some depreciation by comparing blue book values for the same vehicle with various mileage figures thrown in, maybe using 10,000 mile increments, then figure a cost per mile.
The difficult thing may be to work out how much it will cost them if they damage it, or it gets hit by a deer, gets a door ding, etc.