Joseph Manton did business in London, for many years, maybe still does.
A friend of mine, Jon Thomas, can be reached at www.armsport.com
, in Midland Tx, and may be able to make an informed guess about price.
Off the top of my head, I would expect it to be a $750-1000 shotgun, damascus, or fluid steel barrels, more, if tight.
This is certainly NOT a 'trade gun', but the work of a known, and respected maker; don't give it away!!