It can be pretty tricky especially since you can't even see the prices for any of the major wholesalers until you actually sign up with them.
The best distributor I know of in the States is S & S Activewear. As with most big wholesalers, you will need a sales tax license to sign up with them. For Gildan 100% Cotton 6.1oz, it's about $1.65 (including shipping) for a white or natural shirt if you order 50 or more. (Lower quantity only raises shipping costs; shipping for 20 shirts would cost about an extra $0.25 per shirt). They occasionaly run sales which could knock this down another $0.30 per shirt as well.
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