Just as economists fret over GDP, shoppers should worry about CPW (cost per wear)—which is to say, how much use you get out of an item relative to how much it dented your bank account. Those trusty leather dress shoes you lace up daily? They’ve got an enviably low CPW, even if you did plunk down two weeks’ salary for them. Prada’s $1,260 still-life-printed angora sweater? You’ll wear it maybe three times, rack up a few compliments, then get branded “that sweater guy” and shelf it. And that frighteningly costly Pennywise Halloween costume you sprang for? Not penny-wise at all.

Few...