> As with everything, economies of scale and increases in technology will bring the per-unit cost down
"Economies of scale" refers to the various reasons that it's cheaper to do something 10,000 times, assembly line fashion, rather than one piece at a time. In other words, the exact OPPOSITE of what's being talked about here.
It may be useful where , due to the inefficiency of handling an order for one 20 cent knob, the manufacturer doesn't sell parts directly to consumers. The knob that costs 20 cents at scale (on Alibaba) will cost $5 to print. Alibaba operates at scale, and though. 3D printing is for when you're willing to give up economy (pay more per unit) because you're NOT operating at scale.
Advancements in 3D printing technology and competition should reduce costs somewhat. However, costs have already fallen by an order of magnitude or more. It's likely that they cannot be reduced another order of magnitude. The one economy of scale available is keeping the printing machines busy to amortize their cost across many prints, but Shapeway's printers are already busy. Now we can only save shipping costs by having a local machine busy.