There's also the economic rule of "expectations of inflation cause inflation", in that if I think I can buy/sell a lens for $200 in a few years, and it's only $100 now, I'll buy now and either save/make money on the future price. That's how property/stockmarket/tulip bubbles form. Maybe we're in a lens bubble, and the prices will crash in a few years?
In this case there would be an upper limit as long as new PA-145s are available. They're not cheap but surely there's no reason to pay more for an old Polaroid holder.