Just find some interesting articles online about how to evaluate a service loaner car.
Other than the market price, there will be mileage adjustment.
Edmunds says at least 20 cents per mile.
And another article says people also need to consider how long the car has been used. The formula is # of months on the lot * 0.015 * MSRP
So for a loner car (or a demo), the price would be: Selling price for a brand new car - mileage adjustment - age adjustment (maybe?).
I agree there is no universal rule for loaner cars, but something like that might be a reference. Mileage deprecation of a Civic will definately be different from that of a GLE.
Almost none of the BMW and MB loaner deals posted on here reference how long they have been on the lots. It’s more a mileage and end of lease support with those.