Having a 3d printer now, I can get a half dozen iterations on something done in a day and compared to a single iteration per day from the library. So if you're designing your own parts and can't draw them perfectly first go, having the printer is huge.

Replacement battery covers for remote controls. If I value those at $200 each, I've broken even.

If you are lucky enough to have a neighbour to do the commissioning, maybe, but otherwise shipping has gotten so expensive (at least in these parts), and 3D printers so cheap, that a getting few design iterations in your hands will practically buy you the printer.