The source of most of my info on this is the Planet Money T-Shirt project[1]. On this topic specifically, I believe the episode "'Our Industry Follows Poverty': Success Threatens A T-Shirt Business"[2] was the one that covered this (but the whole series, and the podcast in general, is great)

1: http://www.npr.org/series/248799434/planet-moneys-t-shirt-pr...

2: http://www.npr.org/sections/money/2013/12/04/247360787/our-i...