People into muzzleloading, with the exception of a few whales (deep pockets, not big people) are notoriously frugal. You can not devalue something you are practically giving away in the first place. Anyone who is dissuaded by a little engraving, assuming it's good engraving, was never going to buy the gun in the first place at any price so don't waste your time with them.
I'm pretty sure the price of a gun is determined by beer math anyways. Take even the cost of a higher end builder, north of 5k, if you were to pay a plumber for the same amount time as it took to build that gun, you would be paying that plumber millions of dollars, you would never stop paying that plumber, it would just turn into the thing that you are known for doing.