Well, looking through the blue book I don't see any Ithaca M87 listed as a "Deerslayer Featherlight". I see 3 different models called "Featherlight" somewhere in the name, and 11 different models with "Deerslayer" in the name out of the 40+ different M87's listed.
I don't see any listed that came with two barrels. (Deerslayer or Featherlight)
What I -suspect- you have is a Featherweight, and someone purchased the Deerslayer barrel separately. That at least narrows it down to 3 rather than 11!
One of the 3 is a "Ruffed Grouse Society" special edition, so we eliminate that one.
Of the two that are left, a 90% gun in 20g listed from $235 - $500, depending on which of the two it is (if it is in fact either one of them). You could tack on another $75 - $100 for the 2nd barrel.