Hmmmm..... take a pistol. Make it bigger, with a longer barrel, stock, more materials, R&D, etc.
Seems it would necessarily cost MORE than the pistol it is based off, no?
Take a G17 at MSRP around $500. Then design, mold, build stock and take the barrel from 5" to 16"... it should be obvious that it would cost MORE than the underlying pistol if for no other reason than materials (especially given the barrel for a G17 costs about $100 and you're tripling the length for a carbine).
Agreed that a PCC is a very niche market and when you get north of $500, as we've seen, MOST people will just opt for a rifle caliber carbine of a proven design (AR, AK, Ruger Minis, SKS) or perhaps a pistol caliber of proven design (lever actions, or the Beretta). There is a new CZ Skorpion on the market in 9mm coming soon, but will be north of $600.
I do see some benefit for people who are recoil or noise sensitive, reload pistol rounds, magazine commonality, or just want a cool range toy. But that is such a small niche market that I can see why there's little interest by companies in trying to make a sub-$500 PCC, unless it's a loss leader.
Heck, I own an Uzi PCC and it's heavy, awkward, and inefficient and it would be low on my list of practical weapons with that long barrel and relatively low caliber versus size penalty. And it was expensive!!!
But for most people a PCC is just not desirable - you're wielding a rifle and shooting a pistol caliber which is obviously less effective for HD and longer distances/accuracy. Cost of platforms and ammo 556, 762, 9x19, and .40 are all darn close in retail terms. Makes little sense to get a PCC.