Probably about $125-$150 as a mantle decoration.
Janssen Fils & Co was making guns from 1895 to 1965. Since yours is marked in English (Sons instead of Fils) it was made for export to the US and the mass import of shotguns from Belgium was cut off by WWI and never resumed after, so it's a pretty good bet that yours was made between 1895 and 1914. Value will be determined by how nice it looks over the mantle, usually $150 to $250.
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