Dealers always buy at wholesale book and sell as close to retail book value as they can get. They may allow more than wholesale as a trade-in, but you can be sure any additional allowance was offset in the selling price. That means the gross mark-up is the difference between wholesale & retail. As a gross generalization, that about 1/3 the NADA retail book value. Net mark-up is a whole different story, so the dealer has to cover his overhead, repairs, sales commission, the PDI, etc.
2004 American Tradition; 2014 Buick LaCRosse
Homebase in the Ocala Nat'l Forest near Ocala, FL