You are thinking about it in the most rational and simple way. Rest assured.
How do you know you’re getting a good deal? You just compare the Monthly Payment and total down payment from multiple sellers in your region. Get quotes from multiple sellers. Compare the total payments.
What do you do when you rent a car from Enterprise? You compare the total cost to rent between a few different rental agencies, and a few different car classes. You’re not trying to calculate exactly how much Enterprise paid for the car from the manufacturer, the total amount of Fleet incentives, or the cost of the money Enterprise is using to front the car purchases.
Even if you knew that information, it doesn’t help you at all, when it comes to finding the cheapest car rental price.
When I rent a car, because I’m in the business, I literally do know how much Enterprise pays for an Elantra rental, and all the costs that go into it. This gives me absolutely zero leverage when I’m booking my reservation for a compact car at an airport rental location. I just compare a few rental car sites, put in my Costco, AAA, and USAA numbers, and go with whoever gives me the lowest payment. I have the insider information, but using it will just cost mental gymnastics, without giving me any savings, not even $1 off my rental.
Whether you’re renting a car for a few days (daily rental) or a few years (closed-end lease), you just need to compare “how much” and “how long” between different cars and sellers to do your comparison shopping.
You could say “well, if I know the MF is marked up, I could push to lower it, and if I know the selling price is too high, I can push for more dealer discount” - but, honestly, this is just an abstraction layer to negotiating your bottom-line payments. If the price is too high, ask for a lower price. If the dealer doesn’t want to go lower, they won’t, regardless of if you know how they calculated their number.