As to market value here-I can't say, as I have not yet purchased a Model 21, in any gauge or configuration. A close friend from our pheasant hunting club bought a 1939 M21 Tournament Grade skeet gun in 12 gauge-- 28" std matted rib barrels choked WS-1 and WS-2, SST, EJ, PG stock and BT forearm, all original and not refinished- He paid $3250 including shpg., bought in 3 years ago from Wm. L. Moore & Sons in AZ. So, would you pay aprox. $800.00 more from a field grade gun with DT and Ext. that retailed for aprox. $59.95 when it first hit the market. If you paid 4K for it today, could you recover that if the proverbial "Wolf comes to the door" (assuming you had another gun to dispatch the wolf with at hand)??
I buy used L.C. Smiths because there were so many of them made, especially in my favorite all-round gauge, that being 12. I don't try to buy used A.H. Fox guns, because there were way fewer of them made than L.C. Smiths, and I like to have the Laws of Supply&Demand working for me when I open my checkbook.
I would like to own and shoot a M21, but not at these prices. This high $ dealer, much perhaps like Kevins in Thomasville GA. not ashamed of his pricing I should guess, may well be asking such a high $ price because of it's antiquity by serial number. To some "collectors" that is of consideration in the pricing I suppose, just as to others, owning the M21 in 12 gauge given to "Joltin' Joe DiMaggio" would be worth way more than a similar M21 made the same year and month and day, with identical configuration and details (SST, EJ, etc.)
Even if I had $4000 that I could use to wipe my arse with in deer camp and not lose sleep over performing that wasteful sort of stunt, I would pass on this DT 12 with extractors. I have a 12 Parker Trojan SN 211440 made in aprox 1924- the later series w/o the small Doll's Head rib extension- that also has DT and Ext, like the M21 being discussed here, original finish, wood, metal, unturned and properly indexed screws and original BP, with uncut stock or barrels- 28" M&F, that is maybe worth $2k, half that of this M21 ( with possibly refinished wood?) and has the same features, possibly same weight and balance. My Trojan 12 weighs 7 & 1/2 lbs.
Also the Ithaca NID series in a 12 with DT and Ext. "Hell for Stout" like the Parker Trojans and VH(E) shotguns, perhaps not so pricey as the Parkers. Worth your consideration perhaps as well. Just my 2 cents worth.
I have yet to own a complete M21, and if I ever buy one, it will be a 12 gauge, of course, and in the pre- Custom shop field grade configuration. I did buy a M21 barrel set a few years ago, at a gun show (where I sometimes find good buys on gun parts--I am going to keep them, as I am hoping to find a receiver, stock and forearm to fit- I am also hoping to win the Publisher's Clearing House Lottery as well. 28" solid matted rib, choked Imp. Cyl. & Mod. extractor bar or ejector blades missing, take up adjusting screw, and the semi-self-opening assisting spring and cam are in the barrel lug, 90% blue, with some wear at forearm installed position, no dents or dings. SN is 15992. Every one is a while I see ads in the DGJ for "missing guns in parts- mostly WTB" so, who knows--maybe I can put together a "Lunchboxed" 12 M21 and find out what all the mystique is about. I love pre- 1950 Model 12's- and 12 is 21, just reversed, like God and Dog, right???