While I'm sure there's generally plenty of opinions available, on the value of any given item like say, a "Luger" (Parabellum, actually) pistol - the truest indicator of value is what an actual buyer paid cold/hard cash recently for an identical item in like condition.
That's exactly how a realtor determines a value for a home - find out what similar ones in a nearby similar neighborhood recently sold for.
The best way, today, to determine that value (besides putting your own item in an online/national auction) would be to surf online and physical auction RESULTS, i.e. "sold" items, exactly the same as the one you want to determine a value for.
While there are many. many online firearms auction sites, two of the more popular seem to be gunbroker and auctionarms.
Some of the physical auctions that publish results online are named after the founders, like Greg Martin, Julia, and (again), many/many more.
Some auctions specialize in classic firearms, so their results may at first seem inflated, but since they're actual, they reflect both the market and the condition/rarity of what sold.
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