These are sold in large sizes, but in reasonable amounts that won't spoil or go to waste with the average family. If Costco sells it, is it the cheapest that you will get it?
The answer is: probably not.
Sadly, the one thing that Unit Prices on the shelves won't tell you is how the price is affected by coupons. Sales, usually, but not coupons. Likewise, you can't compare the Unit prices if one is on the shelf at Costco and the other is on the shelf at Walbaum's. And the fact that Costco doesn't take coupons (except it's own) tips the balance of the equation.
Now, you can fake it somewhat by keeping track of what should be rock-bottom prices. For example, if you can get Tide for less than $5/100 ounces, that's not only a good deal, it's cheaper than Costco. Use the coupon on top of it.
Pepsi is easier. NEVER buy it at Costco. It's a loss leader for the grocery stores and supermarkets. Stock up when it's on sale for $2.00-2.25 per 12-pack of 12 oz cans.
But one other problem for the bigger is better crowd, which I use with my students ever sememster: which is a better buy - a $.25 bag of chips or a $.50 bag of chips?
Answer the quarter bag gives you more, while they are still available. They won't be around much longer. The $.25 bag has been 1 ounce of chips for years. Wise and the other companies will NOT sell a bag with less. (They tried 25 years ago and gave up.) HOWEVER, the 50-cent bags are down to 1.5 ounces. You get more with two 25-cent bags. Worse yet, the .99 cent bags have dropped from 3.5 to 3 ounces.
There are plenty of other examples that consumers are totally oblivious to. After all, they don't use algebra in their real lives.
Funny you would mention that. I used to get Andy Capp Hot Fries in 1 oz bags for a quarter, or a 4 oz bag for 99 cents. Then they shrunk the big bag to 3.5 oz at the same price.
Like you say, at that price, the quarter bags are a better deal if you can find them.
Last time I saw the big bag it had returned to 4oz with a banner declaring it was giving you "extra."
SD