The price should be on par with other birmingham makers of that era (save for WR, which would come at a premium for birmingham guns.)

The biggest factors for price in a gun of this age are 'condition, condition and condition.' Which is really best addressed on a case-by-case basis.

Caliber is also important in value. Are you talking about a 9.3x74 or a 500NE? Seems like I've seen an Osborne Sidelock shotgun, but suspect that your rifle is a boxlock?

Dimensions good? Anybody been monkeying with it? Bores good? On Face? Oil in the wood?