it's nearly impossible to give estimate w/o inspecting the piece but assuming original good condition (original chambers, no pits or evidence of pit removal by "hone" ie it has sufficient barrel thickness, no stock cracks nor barrel dents or bulges) i would say 900usd+/-50. not having reputation allows me to give an estimate even with next to no info available.
the knockabout is on par with g.e. lewis,...... or one from birmingham bicycle maker called bsa.