What is the value of a 11-87 ducks unlimited 20 gauge shotgun?

Depends on the buyer really. You may find someone that feels having the Ducks Unlimited brand associated with it gives the gun a higher monetary value. Mostly though, the gun is not worth more than a regular 1187. So, I say $300 - $600. Try shopping it around to gun dealers in your area (maybe even a pawnshop if you're wanting a laugh), doing so will give you a better idea the "real" monetary value.