To answer your question; as little as possible
Your price was pretty good for being a year newer, $3900 is about the upper end for a 98 at a private party sale according to KBB and NADA and yours is a 99.
The full price they are asking is 4995 which is near the top according to Kbb and NADA, however I m a firm believer in negotiating down to my financial level plus something more to cover their overhead but not too much more. Their 98 is a trade in with all of the usual value adjustments, appear as a loss on the front end, make it up on the back end. I am looking at somewhere between 4100 to 4400.
Your input is welcomed,