Mostly it depends on whether or not a dealer wants your trade for their lot or intend on taking it to the auction.
If you are purchasing a vehicle with "factory incentives" included, their first attempt will be to steal it, no matter what.
If your trade is a desirable unit given your local, then you have a chance.
There used to be a term used in the business called "ACV" meaning Actual Cash Value, and doesn't have much to do with book values. In most cases this is what a dealer considers the trade value to be if they intend on taking it to the auction or thinks it to be something that will sit on the lot more than 90 days.