The Costco Illusion

Bulk is not automatically cheaper once waste enters the math. A 64-oz bottle of ketchup at $5.99 is about 9 cents per ounce, beating a 20-oz bottle at $2.49 (about 12 cents per ounce).

But if you toss it after using only 30 ounces, your real cost is $5.99 ÷ 30 = about 20 cents per ounce — worse than the small bottle. That is the waste-adjusted unit price, and it is the number that actually leaves your wallet.