I remember needing 4 batteries at some retail store I don't remember, and seeing that the 4-pack was 3 times the price of the 3-pack of the same brand same type of battery. They both claimed to be on sale from some even higher (but more logical) price. I asked the salesperson about it and he couldn't answer the question, so I just bought 2 2-packs.

At least it was easy to do the math unlike just about every time I have to buy bottled water having to figure out whether the 36-pack of .5 liter bottles is a better deal than the 3-pack of 1 gallon ones. Supermarkets usually have per unit prices shown (if you're really lucky, it's the same unit). Pharmacies and convenience stores not so much.