How do dealers make money on box trade ins? It seems like dealers are always willing to give you a nice trade in if you are trading in the same brand of box. Are the margins higher or something on boxes so they can afford give up more? Or does SO allow the dealers a certain amount for trade in credits or something as an incentive to move the boxes?

(And, Unsung, I bet even that HK avatar is illegal to possess in IL, huh?)