So you want to know how to price your T-shirt. That is how much to charge for it. Unless you do something stupid like make a T-shirt with diamonds (it happened "Superlative Luxury" tee and it was priced at $400,000) keep your prices as low as possible. The main reason people buy tees are because they are generally affordable. They pick T-shirt designs that they relate to some how and while that holds some merit and helps determine how much the consumer is willing to pay for the tee, price itself is the main attribute.
Pricing A T-Shirt.
The cost to make the tee and the cost to print the design should be considered even if that is not your end of the business model. The base price is usually set high already. In the POD business model a modest mark up of $4-5 should suffice. If you print your own tees, the lower your production cost the lower the final price should be. Do not look at it as an opportunity to charge even more and make more profit. Keeping it affordable and still making a profit is the way to go. Choose a tee blank that is comfortable, durable, and affordable. Organic is great but it is expensive. Go for the tee blank that are in the middle of the spectrum of quality and price. This way you get a decent product for a moderate price. Most people will not be buying the premium shirts and people that buy the low end tees end up being disappointed and later regret their purchase.
If you are selling your design outright, don't worry about the price of the final shirt as you are selling or licensing your artwork.