What is the value of a 20 gauge Colt shotgun?

I find that $350 is the fair price for a Colt 20 ga, semi auto (made by Franchi of Italy and is almost identical to the Franchi 48). I bought this same gun in 1964 for $267 new out of the box. So inflation and condition should drive the price a bit higher than $350. The least I have seen by gun dealers for one that is in fair shape is $550. I have seen prices, based on condition, from this this fair gun at $550 up to a very clean gun at $1400.