Sometimes, we have to look at the big pictures, the state sale tax has a lot to do with the trade-in value.
It depends on which state you are in. For instance, in Texas where I live, you save a big chunk of sale tax if you trade-in your vehicle because they only tax the difference between the trad-in and the purchase price.
Take the 99' Ody Ex for sale here in Tx as an example: If you can sell to a private party succesfully for $23000 and you are purchasing a similar 02' van for $30190, and the sale tax here is 6.25%, you will scream at the dealer's trade-in offer for $21647. However, these 2 figures are actually the same considered that when you purchase the new van with the trade-in price offered, you pay only $449.38 versus $1886.88 if you purchase it without a trade-in.
So, do not forget the tax part when you look at the trade-in value. Again, it depends on which state you are in.