I helped an acquaintance with her T-shirt quilt business last Christmas. It's always more work than you think.
Don't forget to factor in your time for creating a layout, plus calculating yardage for and time spent cutting compensating strips, sashing, borders, backing and binding. T-shirts are not created equal in that one shirt's logo may give you a 14" square, whereas another may only yield an 8" square. If you have to account for those size differences, it will require more of your time, for which you should rightly charge.