Building my SAS-3 Sten cost me about $800-ish, which includes the Douglas match barrel blank I had to buy ($300-ish) of which, I still have almost 19 inches remaining to play with.
So, let's call that $600-ish I think. These open bolt ones would be even cheaper to build since they have less fiddly bits to build.
That said, it took me in the 100+ hour range to construct the Sten. Even factor that at 50% for mass production (and jigs, etc to speed up production) and you're at 50+ hours per gun. One gun per work week. Ish.
If you're a skilled machinist, and you have a shop to pay for, what are you going to have to charge to keep yourself afloat? Let's say a cheap rate of $20 an hour? That's just about $1000 per gun to build them. PLUS the $600 in parts/materiel.
So, $1600, plus whatever you can milk for profit.
Not to mention the fact that you're building about 4 guns a month at that pace? Hmmmmm.....I don't question his price, I just question the guns. Hopefully the builder isn't using full width sear trip slots anymore.
NS
(Oh, and as a gun-manufacturer....hopefully you have some sort of liability insurance to be able to cover an accident....and the attendant lawsuit.)