Here are a couple posts to get you going:
http://www.quiltingboard.com/main-f1/how-much-charge-t228510.html
http://www.quiltingboard.com/main-f1...t-t249316.html
recent post on general construction
http://www.quiltingboard.com/main-f1...s-t256970.html
Remember there are added expenses with T-shirts such as stabilizer and the extra labor to prepare them.