This is why its so much easier to buy a new car than a used car. You simply have more information on the new car and it comes with the least risk.
You cannot make any assumptions. Dealers get their used cars from trade-ins, direct purchase from private sellers (really no different than a trade-in) and also at auctions. If you attended an auction, you would see what vehicle x sold for - but not the dealer reconditioning expense. You could put yourself in the dealer's shoes and ask "what would I pay wholesale for such a car and still expect to sell at a profit" - but this requires more knowledge than most customers have of the used car market at any point in time and the information is a snapshot that changes quickly. What you would know 3 months ago would be obsolete by now.
In addition, unlike new cars, every used car is different and a product of how it was driven and maintained (and repaired, in the event of accidents).
The best thing to do is to focus less on the price and more on finding the best condition example that you can find and have a pre-purchase inspection performed by a mechanic. Try to get full maintenence records. Once the car passes those tests, negotiate for the best price you can get. "Asking" prices are just that - asking. Having bought dozens of used cars over the last 40 years for myself and my family, I have paid less than 60% of asking price on a great car 3 years old with 25K miles. But I've also paid as much as 85% once. And I've also had very reasonable offers turned down too and then bought new because it made more fiscal sense for that model. Don't begin a negotiation until you are serious, and don't be afraid to walk if the price isn't right.