Well, seeing as they didn't come out until 1937, you're going to have a lot of trouble finding one from WWI...
Good repros, with no trigger disconnect (ie: capable of pump-firing), and all the proper furniture (heat shield, bayo lug, and bayo) can go for anywhere from $500 to over $1K, depending on how well the repro was done. A genuine military surplus one from the Vietnam era will go for over $2K in good condition. A WWII era military issue will go for... Well north of $2K.
They weren't produced in anywhere near the same volume as the Garands (which are north of $1K now), and from what I've read, most ended up with the Marines in the Pacific campaign. The heat and humidity of the island campaigns was conducive to a short life span for a lot of them... They rusted out during or shortly after the war. Just not that many of them survived.
They remained in service with the Marines into Vietnam, the last of the military production in the late 60s. Again, heat and humidity spelled a short life for a lot of them.
And they were never really produced in huge numbers.
So, not many to start with, a high percentage suffered the ravages of the climate in the campaigns they served, means there just aren't big numbers of them kicking around.
They're out there, but there's a lot more well done (and poorly done) fakes than there are real ones around. Genuine ones, with provable provenance, don't come up for sale often, and when they do, if the seller's smart, they go to auction houses where they can fetch the best $$.
I may be way off, and someone will surely pipe in and correct me if I'm wrong. Damn fine shotgun, would love to have one in my cabinet, but I'm just not holding my breath on coming across one at a price I'm willing to pay, at a time that I have the money to spare for it.