Remington's website gives suggested retail prices. They do not list the 11-87 as available in the Ducks Unlimited commemorative edition. They do have the D. U. Mod-1100. See the link below. Used weapon values depend on the condition and age.
100-300 or so
Model 870 Magnum 1982 ducks unlimited edition
Impossible to answer without a detalied description of all features and markings.
Tough question, the Remington model 550 is a .22 rifle not a 12 gauge shotgun.
remington wingmaster 16 gauge
what is a remington 20ga shotgun patented 1891,1903,1904 worth?