I'm sure this question has been answered a trillion times but as a newbie

I would definitely appreciate hearing the answer. For the people who wish to sell their t-shirts to stores, how do you set the price for the t-shirts? I'm asking because I was looking into purhcasing luxe-t t-shirts, which go for about $12-18 a shirt, and wanted to see what a reasonble mark up would be to make my money back from the shirt, designs, tags, & packaging.
Any help would be appreciated. I know there are a lot of tangibles to consider, but if you could give me a ball park figure of how much would be reasonable so that I can see if I have to find a more cost efficient shirt, or whether this is a good purchase.
Basically I want the suggested retail price for the shirts to be $40 - $60 and want to ensure that I get the best material and quality to justify that price.