Nobody has mentioned the Factory Outlet stores in the outlet malls. Nike, Adidas, UnderArmour--I've hit some steals for under $10 for shirts, $15 for pants or shorts--but that's a lucky shot. Most of the time, you'll see stuff at half price. Shoes get better discounts there, too. I'm not sure where you live, but if you are near some of the better metro areas, there are outlet malls around. (I'm in SE Wisconsin--between Johnson Creek and Pleasant Prairie malls. Also I like Tanger Outlets in Wisconsin Dells.) Also, VanHeusen/Arrow Factory stores have good deals as well.
Another is JCPenney--PGA tour for $20 and under. SJB is even cheaper. Check the menswear clearance racks for slacks that will look as good or better than the purpose built slacks...