Think of it this way: If you charge $40 a shirt, you will be making basically $20/hr (at best), and you will be SLAMMED with orders. Say, 40 shirts a week. (Because sewing time is quickly going to be way overshadowed by fabric/button/fabric store time.)
That's $1600/wk. at a break-neck production pace.
Your hands, arms, shoulders, and eyes will quickly tire. The quality of life you have outside of "work" will deteriorate as you spend more time in pain, and less time on social things.
You could charge $100 a shirt, make 3 shirts a day, and still take home the same amount of money without damaging your body, health, or social life.