whatever you want to pay for it...

hard to say, sometimes dealers want MORE for a used one than just buying a new one.

New wings are 20K and over I think base (any rebates though?), so if you can get it for 15K being a standard model, I think you done good IMO. Hard to say what the dealer gave for it on trade in value is the key to finding out your purchase price.
Do you like the black color in a goldwing? My guess is not one of the top 3 color choices but IMO, better than white or yellow and probably orange.
I see locally an old 2001 Wing with plenty of nice accessories with 42K on it very good shape, but is black as well only asking 6.9K. Tempted to jump on that one since well over 1K in useful accessories like driver backrest, passenger armrests, 6 disc CD changer, hwy pegs, chrome rear luggage rack, heated grips, and newer vented windshield, and blacked out full baker air wings, and some chrome add ons as well to break up all that black. It looks sharp in pics have not seen it in person yet though if still for sale locally.