To reiterate what "bythesea" is saying. It is all about perceived quality. If you expect your t-shirt to be a $15 item, then that is all it will be. In my niche my very basic Gildan shirt sells for $20 and when I started out with them priced at $15 buyers strongly advised me that I was asking too little and people would start looking for faults in them because of that, all based on the environment in which I market and sell my products. $20 to $30 for my t-shirts seems to be the sweet spot and I never anticipated that (woohoo). Be conscious of your target market, do your homework up front and be confident in your product.