People here in Cincinnati are charging between $12-15 a shirt, so for 12 shirts, $144. As a photographer with her own studio, it is the industry's standard to charge 3x what our material cost is. The theory is 1/3 is to cover materials, 1/3 to cover taxes, insurance and overhead, and a 1/3 to pay yourself. While you don't have taxes, insurance and the like, you DO have additional wear/tear on your machine, additional electricity costs, and your time, so I would charge the cost of materials plus maybe even 50%. I'm not sure how close that is to $12-15/shirt, but definitely make sure you break down the costs to cover what you're putting in!