I believe there is a larger than 5% difference between an unfired in the box with all the accompanying stuff over a fired and well maintained one.
I could be wrong, but there are enough hard core collectors of museum pieces that if it is something they need to fill a hole in a collection or just to scratch an itch, they will pay a premium for it.
I have owned several unfired guns over the years. I do not shoot them for one reason and that is because there are people in the market to whom a gun like that means a great deal. I can always get a shooter grade to scratch the itch to shoot it if I want to.
The number of unfired guns from the 60’s and 70’s is shrinking every year. Either because somebody shoots them or someone buys them and pigeon holes them for eternity in their collection.