Why are they expensive?
Two reasons, really.
One is that making those barrels is not easy. You need a lathe. You need to know how to use a lathe (which is much more expensive than the tool). You need stock to turn. Then, you need the most expensive bit of all--research data. Like not merely the dimensions of the "real" thing (remember that the barrels have some taper, and ofter a curved or stepped taper). But, you also need to know what the average diameter of the mantlet or gun base in the Trumpy, Tamiya, or whatever kit you are making it for, too. (Probably a real good idea to know if you ought recommend to the buyers to epoxy or PVA a bunch of shot into the gun breach in the kit to balance the tube you are selling.) Then there's the whole tiny-business economy expenses just to run a cottage biz, too. Wret5ched devilish things like taxes, fees, overhead, advertizing and the like.
The other thing is your personal economy. We only have nn hours to devote to hobby time. At what point is the time expense in puttying, sanding, seam-filling, etc. not a good use of those hours?