What Makes a Gun Valuable
The value of a gun is dependant on the type of firearm, the condition, and the rarity.
Type of Firearm
The type has to do with what firearm individuals want. Currently the most valuable guns are original 1911s, Lugers, and any guns used by the United States military. A pistol like the original Generation 1 Glock in unfired condition has a high resale value but only in that condition.
Condition of the Firearm
The most collectable firearms are unfired in the original box with all the paperwork. Many collectors want all of the original material that was shipped with the gun and the lack of the original box can reduce the value also. Wear, rounds fired, aftermarket additions, and replaced parts all have an effect on the value.
Rarity
The rarity and the price are closely tied to the type of firearm. Just like there are cars that are valuable only for the rarity and not the actual mechanics. The value of guns has as much to do with how many other guns of that type are available to purchase. Original Colt Single Action Army pistols where very common, but after a 100+ years they are worth a premium.
What is the most expensive gun?
The most valuable gun in the world is a .45 ACP Luger serial number 2, which was part of a two gun set that was sent over to the United States Army for testing in 1906. The last time the firearm changed hands the price was 1 million US dollars. There are other famous guns that are in museums that are not for sale. One example would be the pistol that was used to kill Lincoln.