You need to know what your fixed and variable costs are. Fixed are things that do not change like rent, ins, etc. Then divide that by the average number of shirts you make a year. After that make a spread sheet of how much time it takes you to set up a job, 1 color, vs 2 color, etc. Then calculate how much time it take to print. After you know all those costs you can just plug in your profit that you need to make per hour and make yourself a cost sheet. Then its really easy to display to your customers based on quantity .
lol I hope that all made sense??