KEH (which I'm sure you already checked) is selling that lens for $7500. The Nikon site has the AF-S 500mm F4G ED VR factory refurb for $7700. Obviously at the end of the day it's all based on what someone is willing to pay. I would guess $3500 if you're selling to a used camera house or a shop and about $7000 to a private party.
If it is the earlier non VR version, and in good shape, they run about $2500. If beat up, hard to get $2k and, if super clean, tops out at $3k. If indeed a VR, might have two versions. I and II. Those add $.
This should tell you via serial number which you have
4 different possibilities. 500mm f4 AFS versions I, II, VR G, VR E
the lens nameplate has it all, and lens case also should on first two models. I think the 2 non VR versions had the hard, pale tan suitcases. Not sure on newer VR versions, they went with nylon ballistic black field case I bet.
all lenses should have separate large lens hoods.
lastly, even if an older version, if lens and case are really pristine, it might have higher increased value as many people have pet lenses, like cars, where a clean example can fetch top dollar. But clean must be immaculate.