Tough question, as it could encompass holsters used by famous persons on historic occasions.
What if you could purchase, with rock solid provenance, the holster worn by Sir Winston Churchill during WWI, for his Colt Govt. Model .45? The holster for his Webley-Wilkinson when he was "Mentioned in Despatches" for gallantry on the Afghan frontier? The holster for his 7.63mm Mauser, used in the famous cavalry charge at Omdurman?
The holster worn by Sgt. York as he perfomed so valiantly while earning the Medal of Honor? The one used by Capt. C.H. Upham, VC (and Bar), the New Zealander who was one of the three men in history to receive the award of the coveted Victoria Cross for a second time? What value does the West Point Museum place on Patton's holsters and belt? On the holster worn by Reichsmarschal Goering when he surendered with his S&W M&P .38?
We'll probably never see the holster worn by Maj. Erich Hartmann as the top-scoring fighter pilot of all time, with, I beleive, 352 kills? I think he wore a Walther PP .32 and insisted on wearing it in Hitler's presence as he was awarded the Swords or Diamonds to his Knight's Cross.
What about any holster worn by Frank Hamer or by Tom Threepersons? Even those owned by famous gun writers like John Wootters or Skeeter Skelton? By Elmer Keith?
What would the shoulder holster worn by Maj. Thomas McGuire, MH be worth, if he hadn't been wearing it when he crashed in combat? That worn by Gunnery Sgt. John Basilone, MH on Guadalcanal? Was the holster of Maj. Richard Bong, MH retrieved and kept after his fatal crash?
The most expensive holsters that I own are from El,Paso Saddlery, basketweave-stamped and lined. What I really love to find is older Bianchi amd Safariland holsters in a bin at gun shows, where they are sometimes bargain priced as just old holsters. I got a few for under $20! Condition on these was about new.