Boyds sells stocks for the American Knight which will fit a MK also. Its my opinion this has driven down the resale value of MKs with laminates a tiny bit.
Right now i know where to buy a like new MK85 SS with a composite stock for $225 shipped. Its a 54cal so that hurts it a bit on resale.
Very early MKs with a Lothar, Numrich or Wiseman barrel barrel might fetch a little more than a GM barreled MK if you find the right buyer. A Lothar barrel 45cal MK85 being one of the hardest to find at a reasonable price.
One other thing that might effect value is barrel length. If i remember correctly most MK had a 24" or 26" barrel but there was also the Back Country Carbine or Light Knight with a 20" barrel.
Here is a good read on older MKs.
http://www.blackpowderjournal.com/archives/vol2no3/articles/BPJ23-2.htm