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This info is a bit out of date and American but it may give you some idea.
From the "Standard Catalog of Firearms":
Exc = $900 V.G. = $750 Good = $550
"An original holster is $75 to $150. "All 18" barrel variations are rare".
"The Model 1921 was originally designed for the 2" .410 shotgun shell, but Marbles changed the extractor markings on some guns to 2 1/2"to indicate factory rechambering for the 2 1/2" shell , which has been reported to have started in 1924".
My understanding is that these are Restricted firearms. I think the numbers listed above would be excessive for something that amounts to a Range-Only curiosity.
American values mean nothing here. They're worth far more Stateside($1500 to $1800US plus, according to the assorted auction sites) than here due to being restricted.