More pieces of clothing cost less to make, and less pieces cost more. A single item clothing sample that costs $13 to make in the USA in bulk production (300+ pieces) for labor only costs $50 to make one sample of. Why does garment manufacturing work this way?
Most anything you make, whether it’s printing a book or making coffee mugs, is cheaper in greater volume. The short answer is set-up and staffing, or overhead. It takes a lot of people and a lot of machines to make a sample. The cutter can lay out individual pattern pieces on top of one layer of fabric and cut it out with scissors to make one sample. The same cutter can also cut, or ban saw through 10 or 20 layers of the same fabric in the same amount of time. The machine operator or sewer takes time to set up multiple cones of thread on a serger sewing machine and make sure it’s running smoothly. It takes time to set up the snap press, put the right feet and needles on the other sewing machines, make sure the irons or steamers are running correctly, etc. It takes the same amount of time to do the machine set up for one sample as it does for 300 samples. Think of all the hourly wages that go into that preparation time.
Another factor when it comes to making clothes is sewing efficiency. When you sew something new for the very first time, you must figure out the fastest way to put it together, follow the tech pack, cross-check the sew-by sample and make sure everything is lining up correctly and neatly. When you make the same thing over and over, it gets much faster to sew so efficiency goes up. Greater efficiency equals less time going into sewing one garment. Less time equals less cost.