It only adds value if your gun is mint.
Having an old firearm "new in the box" is what a lot of collectors go for in terms of investments.
Used in the box won't add much, and definitely not enough to offset the purchase price of that box ($400).
If the gun is used (and shows it), then it's not as attractive to the kinds of collectors who value having the box.
Keep in mind, those boxes were serial numbered to match the gun, so even with the box, if the number doesn't match, it won't tickle a high dollar collector's fancy.