Here's the jist of the math:
“Before buying anything, consider its so-called “cost-per-wear.” For example, if a pair of jeans cost $200, but you plan to wear them three times a week for a year, your cost-per-wear will be about $1.28 each time you put them on during the first year, and less if you keep them longer. Meanwhile, the cost-per-wear of a dress you bought on clearance for $40, but only wore once, will be $40. The math clearly tells you which is the better deal. So make note of what you wear the most and spend accordingly.”
I don't ever figure out a cost-per-wear exactly, but I do think about it. If I don't think I'll wear it often, such as going out dresses, then I'll usually just go to Forever21 and buy something cheap.
So, how do you shop? Do you try and figure out how often you would potentially wear it?