in his defense, you should be able to give him a ballpark value from Manheim. As for payment (only morons shop that way); the finance manager should be able to tell him what rate he could get if he has > 720 credit. With that, its a quick calc.
99.999% of the time, we walk in with a pre-approved loan from the credit union. I've also gotten a verbal value on a trade BOTH times I traded a car this year. On inspection, they had no issue standing behind that and it made my life much simpler as Carmax is a PITA and lowballs anyways.
In other news, my old Benz is still for sale at the dealer I traded it at....I think they gave me too much for it and can't get out of it now.