Bost is something of a mystery. Little is documented about this maker, and it is not listed in Wieland's book. You see them occasionally, Wholesale in Edmonton has a single barrel sidelock trap gun by Bost sitting on the shelf. They are asking in the neighborhood of $1,300 for it. It has been there for quite a while now.
Sigismund, I am curious about the gauge of your gun. The barrels show a bore diameter of 19.3 mm, which is 0.7598 inches. The nominal bore diameter of a 12 gauge is 0.729 inches, while a 10 gauge is 0.775 inches. Either the gun is a very large 12 gauge or a tight 10 - I'm betting that it is a 10 gauge. The beavertail fore end, semi pistol grip and bolstered frame are consistent with a Spanish 10 gauge. The muzzle diameter of 18.5 mm translates to 0.7283 inches, providing 0.0315 inches of choke, pretty near full choke on a 10 gauge.
If this is the case, you have a small problem - the gun was not designed for steel shot, and virtually all commercially available 10 gauge ammo is steel shot. Options include hand loading bismuth shells, or opening up the chokes and hoping for the best.
If the gun is indeed chambered for 10 gauge shells, then the resale value is hurt by the lack of available ammo. $250 seems to be the maximum that these guns will sell for around here. If it is a 12 gauge, then Claybuster is most likely correct.
Edited to add: I am blind. The water table shows the gun as being chambered for the 3.5" 10 gauge.
Sharptail