You're going to need a 150 pound piece of steel to start with IF you can find 4.5" material. If only 5" material, 180 pounds. Surplus stainless will cost at least $1 a pound, maybe more depending on where you are. New material probably twice that.
Then figure in the labor. How complicated a design? A design with a lot of fillets and astragals and an echinus is going to take a lot more time than something with mostly straight lines. And if the maker is trying to eat from selling cannons, he can't do it at 5 bucks an hour.
I figured once that I had almost a hundred hours in my 1/5 scale 24 pounder for just the barrel.
"Fair" is a word that has been mightily abused in the past several generations. In a free enterprise society, a fair price is one that both parties agree on.
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