Henrique, if you are sure it is a live pigeon gun (you can see that because the gun have big fences and almost all pigeon gun have side clips) then the asking price is realistic.
This depends de condition of the gun of course and how many money you gona spend for the restauration.
About the connection with J. Defourny I don't know if there was a connection the water table of the gun is stamped with the Defourny trade mark.
Marc Gettemans