We can help you--but need a lot more info! Field gun or skeet gun---condition--alterations---refinishing---serial number. There are more "made up" 28ga. M12's then originals. I've had at least 10 of them and one truly original skeet gun. Have you pulled the stock? Is the back of the receiver marked "28"? Give us some more to go on and we will jump in. Or email or call me directly if you wish. Bill 609-658-7739
If its 100% original depending on the choke, your looking at not less than $4K and as much as $7-8K in todays market. But the gun has to be spectacular and 100% original for that money. I have 4 of them and have turned down as much as 7K.