they cant place jewel vendor per say . as a faction town finance minister you can place reg, bottle, boards, and ore vendors. you can also place a horse breeder that sells faction horses but thats a seperate option.
each of those , as well as every npc vendor price in that town, is controlled by the tax you place on your members . so yeah if the whole town is cheap the npc jewelers will be cheap too .
if a town is cheap it means that faction is getting no taxes from that town ... thank them. even minimal taxes would make the price the same as a non faction town .