Just like any other used item, a original Colt pistol is worth what you pay for it. And, just like other antiques, the value of a original Colt revolver is set by it's condition, rareity and prominance. Every German Luger I have seen for sale was Adolph Hitler's personal sidearm.

Rarity is the main factor. For example, a .45 cal. German Luger was auctioned off for $2,000,000. There was only one made.