But here's the point: It is hard to tell who's winning there without a full comparison of your pre-Costco shopping habits and extra costs on vehicle wear and fuel for the added miles. And Costco wants it as hard to tell as possible. Amazon uses the same basic formula. Which is why I rely on the research.
As with anything, we need to know prices. No store cares about my best interests. In the USA at least, most folks have a calculator in their pocket, so figuring out unit prices doesn't take too much effort.
We shop Costco with the understanding that not everything is cheaper and not everything is a good value. We don't buy anything we are not going to use, but do buy in larger quantities. The price differential for the jumbo jar of minced garlic is enough that I can throw out 3/4 of the garlic and it's still cheaper than jarred or fresh garlic. We eat a lot of vegetables, so a 6 count pack of sweet bell peppers is a large savings.
Meats are not cheaper, but Costco tends to carry better quality, so it's a trade off that we are not buying the cheapest meat.
It's not unusual for us to walk in for 2 items and walk out with 12. But none of it is junk and it all gets used. We would have purchased these things elsewhere, and Costco was a better value. [Impulse shopping is fun, as long as everything is honestly needed.]
The rebates on purchases and the Citi/Costco Visa card are significant ... if you use them. We get 4% back on all gasoline purchases with the Visa, not just Costco. I tank at Costco (when convenient) as the price per gallon is $0.04 to $0.30 less than other stations, not including the rebate.
As a child, I recall my parents scanning the grocery store flyers, looking at prices. They'd map out which stores they were visiting that week, depending on what was needed, what the prices were, and where the stores were in relation to each other. I learned to comparison shop, and to get the best value, which includes wear-n-tear on the vehicles.
All this said, a lot of people do over-shop when they see pretty things on the shelf ....