Sometimes you can find racks in the back that have clothes that aren't in season for less than $7. For the most part, their camisoles, leggings and socks are all under $7. And like joe1234 said, you can find good yellow tag racks; I actually found a $6 skirt last week!
Just keep looking around at forever 21 you can find cheap sales (shirts,socks,ect.)ometimes there marked down the yellow tags are the sales you can find clothes cheap there i kno i do.
It mostly is jewelry and accessories that are that cheap at forever 21. I guess if you keep a close eye out for sales, you might be able to find a piece of clothing around that price.