It is a black powder ctg. rifle. To answer your question, it is 'about' $2,400. It is not a trap-door as in the 'list' of description but is a rolling block as noted in the botton descriptor paragraph.
; It is 'about' .62 claibre. 20 bore is .615", but could be anything up to .630" groove diameter depending on the depth of the rifling. This will be important when fitting ammunition (bullets, brass, etc) to it. The chamber will have to be cast in cerosafe or other such material to find it's exact dimension so ammo can be custom made for it.