If you carry a cell phone, you also have a calculator. Pretty much every mobile phone on the market has one, so you have no excuse not to do some intelligent per-unit math while you're out shopping.
Companies love that their customers assume economy of scale, and they want you to assume that buying more stuff means you're getting a better deal for it. Too often, though, it's not true.
This summer, WalletPop made the papers when we wrote about the "great McNugget caper," which proved that McDonald's popular bird chunks can actually cost more if you buy them in bulk.