One thing to call out is that costco.com and in-person have different offerings (& prices) -- but you probably know that already.

I just dusted off my spreadsheet, and it's not as complete as I'd like it to be. I didn't normalize everything but did have many of the staples like milk and eggs normalized; some products had multiple units (eg, "bananas - each" vs "bananas - pound"); and a lot of my comparisons were done based on the store (eg, I was often comparing "Potatoes - 20#" at Costco but "Potatoes - 5#" at Target over time).

Anyway, Costco didn't always win, but in my experience, they frequently did -- $5 peanut butter @ Costco vs $7.74 @ Target based on whatever size and brand I got, which is interesting because Costco doesn't have "generic" PB, whereas Target has much cheaper Market Pantry, and I tried to opt for that.

My family’s favorite experience has been that Costco usually doesn’t have the cheapest option but it has a good value option.

Our main example is something like pasta. Our local grocery stores all carry their own brand of dirt cheap pasta but it’s not as good as the more expensive pasta at Costco. Comparable pasta at the local grocer would be more expensive.

For items that are carried at both stores, Costco is usually no cheaper than the regular retail price and rarely much more expensive.

The quality difference I find between Costco and Walmart is significant, even if the price is not that different.