Hmmm... I usually don't wander into the "how much is it worth" territory. With these, I'd say considerably less than the correct set with the correct papers. You can do an eBay ended auction search and see what the "right stuff" sells for, though with them being such a limited production, none may show. Another option is to ask Bernard Levine if he has a grip on the market for these. If you could buy the display alone for $100 (if it is perfect and complete except for the missing knife) and then add the correct knife at a future date for a reasonable amount, it could be a decent deal. Most of the original DU decoy sets retailed for $350. The SDU77 alone (2002 DU birdhook) retailed for $42.95. I don't have the MSRP for the DS decoy set.
Michael