Some things that are cheaper at a low scale is quite expensive at scale, 3D printing is obvious here, your way to consume less 3D printing may be opposite to the way that regular plastic manufacturer does, so you need to adapt your process to the process of your suppliers.
I wasn't even touching manufacturing at scale, because sometimes you really do need just the one. But it should not be so fragile you can't carry it from the garage to thermostat mounting position.
I learned that the hard way when I automated the heat lamp that I put in my chicken coop. Having to noodle around with screw terminals while being pecked at by an angry rooster was not a great time.
As someone who sells a low-volume niche product (as a sideline), the problem isn’t that 3D printing costs grow per-unit, but rather that they don’t meet people’s intuition of what plastic things should cost.
A box that might cost $0.25 if injection molded might be $25 if 3D printed.
