It must be worth $600.00, that's what you paid for it. The only 16 gauge Model 12 we have is my Dad's gun, manufactured in 1958. Dad added a poly-choke to his.
That shotgun lives at my son's house now, and if you asked him what his grandfather's shotgun might be worth, he'd tell you that it's priceless. It still barks and roars over the pasture, here his wife is shooting it during our family skeet shoot a couple of years ago.
The photographer caught her in recoil.
$600.00 is not a bad price for a pristine Model 12 and I've seen them go much higher than that. Condition is everything in a shotgun. Enjoy your Model 12, they're great shotguns.