Best way is to find out what people actually sell them for. Once you know this, you can add some profit for the Dealer and get a good idea. A quick look at Autotrader shows (53) 2004 E55's for sale, with only one by private seller. Asking prices range from low 70's to low 80's. Interesting that there is only 1 non Dealer selling their E55. Nothing else out there that can deliver the performance at this luxury level.