When Comparisons Contrast

I can name any number of accounts where charges are all over the board. So what is the right number, or what is the proper accounting method? If I'm comparing two dealerships' expense percentages and the expense charges differ, is the number credible? What about dealer trades? If you sell a vehicle that isn't in your inventory, chances are you will obtain the vehicle from another dealership. When this vehicle is brought into your store, do you send it to the service department for a pre-delivery inspection? Chances are you do, but where is the expense of that inspection charged? Is it to the vehicle as a cost-of-sale item or is it charged as a delivery expense? The top I'm referring to is the gross. If expense charges aren't consistent, it's not a valid comparison.