Not sure where Concrete got $400, but I went to gunbroker.com and check the closed auctions. The basic guns ended their auctions for $171 to $262. Then you have to do the add on's. Not sure the brand on the stock but I would say $100 or so.
I can tell you that I just sold an 870 Magnum with a 3 shot ext, a Knox spec opts recoil suppression stock, costs $175 new, Pic rail mount for a light laser and a Knox fore grip and Knox power pack (total add on's cost me over $300) for $450. Plus I had the original wood and all parts that came with the stock gun.
sorry i should be more clear blue book values for the 870 models msrp ranged $350-$425 so i just went in the middle so he had some what of a starting point.sorry for the confusion