When trading on a new car, the trade in they give you is actually coming of the retail price of the new car. It just looks more appealing to you as the buyer if they give you a better trade in value. Demonstrators are the best for this because the dealers can write off large chunks of the value as advertising costs...
It's an interesting game... Best I could do on my VX S 4auto Sedan if I was selling it would be $12,000 - $15,000 (trade in $10,000) but because I want to trade on a VE the dealers have offered me around $13,000 trade in. I don't care too much... I usually talk in terms of "change over price". Makes it easier for all concerned and quite frankly the dealer can churn his numbers on whatever side of the deal he wants, as long as I get the change over price I'm after