About $400 to $600 depending on condition, where in the country you are trying to sell it, and how bad a buyer wants it!
If that is the gun it looks far better than most Model 12s
If you are asking for insurance purposes declare it at $400. The only time value matters is at the time of loss, and then they will pay whatever it would cost for them to replace it, regardless. All you are doing when you declare a value when adding any item to a Personal Articles Floater is telling the company how much premium to charge for it. I wrote Personal Lines Insurance for 25 years until I retired in 2019.