I think it also depends on HOW it is engraved on the gun. If it was done with an electric Vibra-etch on any exposed area, it'll be worth less unless the gun is somehow historically significant and the Vibra-etch is part of the story (highly unlikely, but then stranger things have happened). If it was done by a very famous engraver and done very tastefully in terms of location and style, it may or may not add to value for the reasons others have stated.