[Bruno]

Given a TL 3-4 world where everything is hand made one-offs, you may not get charged a mark up at all.

Instead, you'll likely get something Cheap for standard cost, if the person making it isn't any good with your body shape and doesn't have the skill to figure it out on the fly. If they're ethical, they may charge you less, knowing that they're producing a kinda shoddy product.

If they're excitable and curious, they may do it at cost for the chance to learn how to make a breastplate for a winged kobold (for example) - but you could end up paying for two or three times the normal materials as there's a few failed experiments. And waiting a long time.

Manufacturing at this TL is a personal relationship, and it's going to depend on the people doing the work.