I don't know what the guns are going to cost, but for the time both guys (metal and wood work) are evidently putting into the project, I'd guess that they're going to be asking a price that more than justifies their efforts.
I agree about the barrel for the field carriage: It seems to me that a model of an M1841 6-pounder mounted on a CW carriage would definitely be more appealing to potential buyers, than a barrel that looked "similar" to an M1841 6-pdr.