Then the cost of it moving

Per rental day, roughly:

• Fuel, if you supply it. Most self drive operators do not, since the customer returns it as they found it, but with driver work almost always does.
• Servicing and tyres, spread across expected mileage. An oil change every 5,000km divided across the days it takes to do 5,000km.
• Cleaning between rentals.
• Driver pay, for with driver rentals, including their meals and any overnight allowance.
• A repair reserve. Something will break. Setting aside a small amount per rental day is cheaper than being surprised.

Call that your variable cost per day.