If you're trading in, you'll either get a good value for your old car, OR you'll get a good deal on the new one. You seldom get both, unless you're trading in the same brand. You just need to have an idea of what balance you want to pay.
For example, if you're trading in the Range Rover on an M6, the RR plus $75K could mean you're paying full boat on the M6 but making out like a bandit on the RR, or you're getting a great deal on the M6 and getting raped on the RR. It's the same money either way you look at it.