basically you should double your production costs. stores will want to be doubling from wholesale to retail to sell them.
for example my blanks cost around $7 each,then between $3.50-5.50 to print, so say $4 on average which is $11 to produce a shirt, then i double my price to $22 to $22.50 to sell to stores,in turn they will then set a retail price of about $45
*thats based on Australian prices so obviously if your from the US it will be different
also you got to look at what other start up companies are selling at,how good a product you have and other factors. Also look where you can bring down your overheads like printing and garment costs etc. buy and print in bulk if you can.
retailers will always push for the lowest price they can get,to help counter this you can offer price breaks. eg. 1-15 shirts at X amount, 16-30 shirts a dollar or 2 or whatever cheaper. then in turn you can go to your printer or shirt manufacturer and your able to order more which should bring your overhead costs down.
anyways, basically you want to double what it costs you to make the shirt in the first place and sell it at that price.stores at the very least want to be able to sell it at twice the amount they purchase at. this is a general rule and can vary.