Well, that's one useful thing the internet's good for, getting an accurate trade in value for you car.
You can arm yourself with this information printed out when you make a deal, and get to the trade in value part.
Albeit I've never owned an expensive car. However, when I bought my current one, and traded in the old, it was really helpful.
When we got to that part and he said a figure, I said "I'm not happy with that amount." He asked me what it would take, I said a number, he did the "let me speak to my manager" thing, came back with "ok", and I drove home that day in a new car.
Didn't need to show the estimate, because he knew it was fair.