Well yea, thats economics. Your time is worth the next best thing that you could be doing with it. Everyones time is worth something different. you should price your product so that the time (and stress) that it took for you to prouce the shirt equals the amount that you are selling the shirt for to you.
Most people are selling shirts for around $10-$15, I would say. You have to look at the cost of the shirt, and the cost of your time. If you feel like you would be happy to sell the shirts for $5 each, go for it. You would probably sell alot of shirts at that price, although you probably would not be maximising profit.
The ideal is to find that equilibrium, where supply meets demand. That is where the profit will be maximised. It seems like that price is around $10-$15 judging by the prices that most people are selling the shirts for.