folding stock tends to boost your price up. Two reasons: One is the cost of the folding stock (duh), and the other is that the folding stock is often only an option on the more expensive footprint frames. Kinda like Audi's all wheel drive. They could put in on a Jetta, but they don't. You have to buy an A6 or whatever to get that option (sorry, it has been a while since I car shopped, I might have the analogy all messed up).
You mention MDT. They make a really neat product. But, they are by and large keeping costs down by mixing in common AR parts, right?
I am a dealer for Cadex. I really am biased, because it is such a great platform. But, for example, Cadex has a great folding stock lightweight platform. The Lite Competition is lightweight (thus the name) and also has a folding stock. List price is "up there," but not as high as other platforms, both Cadex and others. Cadex also makes the Urban Strike, which is half the price of the folding stock model, as it, too, is using AR platform parts, like MDT.