There are a number of fairly decent Model 12's in 16ga on Gunbroker with a "buy it now" price in the $400 to $500 price range and the ones with auction prices starting in the $700's not getting any bids.
If the stock has been changed, different wood(unless higher grade added), pad added(if cut) and Polychokes or Cutts Comp. will bring the price DOWN. Ribbed barrels, factory checkering, upgraded wood and factory upgrade models will bring the price UP
Go to the Auction and Dealer sites and see what they are asking and how the condition of your shotgun compares. Asking price and selling price can be quite different, some times these guys will hold a gun for years either hoping they will get the right buyer or dropping the price every few months. I just bought a sxs was listed few years ago and I got it for 2/3rds of the original listing price.
Gunbroker
Gunsamerica
Gunsinternational