For screenprinted shirts there is a very substantial setup portion of the cost though, the actual shirts are dirt cheap in any case but the setup for the print

For screenprinted shirts there is a very substantial setup portion of the cost though, the actual shirts are dirt cheap in any case but the setup for the print run can take a worker a couple of hours for multiple screens. The result is that for small runs the setup cost dominates. Leather jackets are just expensive to begin with, so I would expect the item cost to dominate even for small order sizes, resulting in a much shallower discount curve. And even if the jackets were customized they were probably embroidered or had embroidered patches applied - the machines used for this are computer-controlled and there is essentially no setup cost involved (besides some time to prepress the art).