the very high prices are for the big Nitro Express only
I think there would be no difference between a Sauer&Sohn 450 BPE and a british 450 BPE beside of the fact that the british double rifle was maybe build in Belgium.

ok, you know how I mean it
the most valuable double rifles are the one made for indian maharajah's when money was absolutly no question. this were safe queens also the most time! the price of a double when made in the golden age will reflect to the price on the collector market today.
but there will allways be a big difference between a Nitro express double and a Blackpowder express double no matter how useful a BPE double rifle could be on most game.