Sale signage is optimized to catch attention, not to communicate unit economics. "Buy one get one 50% off," "3 for $10," and "Save 25%" are all framed around the transaction, not the per-unit cost — and per-unit cost is the only number that tells you whether you're actually getting a deal relative to the other options on the shelf.
Worked example: the discount that isn't
Example only: Product X is on sale, "25% off," bringing a 500 g jar from $6.00 to $4.50 — that's $0.90/100 g. Right next to it, Product Y, full price, is a 675 g jar for $5.50 — that's $0.81/100 g. Despite the flashy 25%-off sign, Product Y at full price is about 9% cheaper per 100 g than the discounted Product X. The sale sign is accurate about the discount on that item; it says nothing about whether that item was competitively priced to begin with.
Sale item vs. full-price neighbor (example)
| Product | Status | Price | Size | Price/100 g |
|---|---|---|---|---|
| Product X | 25% off | $4.50 | 500 g | $0.90 |
| Product Y | Full price | $5.50 | 675 g | $0.81 |
Decision rule
Decision rule: treat every sale sign as a prompt to check unit price, never as a substitute for it. A discount percentage tells you about that item's price history, not its price relative to competing options today.
Multi-buy deals hide a second trap
"3 for $10" deals often require buying the full multiple to get the discounted rate — buying two when the deal requires three can mean paying full, undiscounted price per unit, not a prorated discount. Read the fine print: is it $10 flat for any quantity up to three, or $10 only if you buy exactly three? If you don't need three, calculate the single-unit price both with and without the deal before assuming it's cheaper for your actual purchase quantity.
Sale cadence and stockpiling math
For shelf-stable goods you buy repeatedly, the more useful comparison isn't today's sale price against today's shelf price — it's today's sale price against the item's typical price history. If a product regularly cycles between $5 and $4 (a 20% swing) and you buy it only when it's near $4, your effective long-run price is close to $4 regardless of what the "everyday price" tag says. Tracking a handful of routine purchases over a few months tells you the real cadence far better than any single trip's signage.
When a sale is a genuine deal
Sales are worth acting on when the resulting unit price beats the best non-sale alternative available to you, and when the quantity discounted matches what you'll actually use without waste (see the bulk-buying math for how to handle that). Both conditions have to hold — a great discount on a size you'll waste half of isn't a good outcome, and a mediocre discount that still beats every other option on the shelf is fine to take.
- Always compute unit price for the sale item and its full-price neighbors before deciding.
- Check whether multi-buy deals require the full quantity to unlock the discount.
- For recurring purchases, compare today's price to the item's normal price range, not just its "regular" shelf tag.
- Confirm you'll actually use the discounted quantity without waste.
All figures in this guide are illustrative examples, not current prices. Cotco does not sell products and is not affiliated with any retailer. Verify prices, sizes, and product details with the seller before buying.