I think other factors come into play than just the calibre
and the makers name.

Type of gun (ie Royal or Modele de Luxe),
Year made
Quality of Build (not all "BEST GUNS" sow what I call "build quality")
Fit, feel and Balance

I would say a Purdey 600 is worth more than a 577 H&H, with roughly
all lse being equal.

At that level of gun / price, you either want it or you don't, the people who
buy these guns have alot more money than the price of the gun.

500 Nitro