Make a round about quote and learn from the first job. For us, 12 shirts single color front and back usually runs $ 12 each. At 24 that price would drop to like $ 8 I posted a pricing spreadsheet a while back that could help. Mainly you want to make a profit on the shirt, materials consumed and make a resonable hourly wage for yourself... and the shop if you have overhead. Mark your shirts up 100% that's a given (unless you're paying retail... you need a wholesale supplier) Then figure your time at $ 40 - $ 60/ hr for art, coating/exposing screens, printing shirts and clean up afterwards.