Costco is not in the same business as WalMart. They have an order of magnitude fewer SKU's. They deal with a completely different clientèle, and the profit for each item is much higher, so their labor costs are lower.

You can not compare WalMart and Costco.

Except that, if you have both a WalMart and a CostCo in your town, you'll still make a cost-comparison, between:

    cost of set of things found at WalMart A
    + cost of a few extra things not found at WalMart B
    + travel time T

and

    cost of set of things found at CostCo A'
    + cost of a few extra things not found at CostCo B'
    + travel time T'

In other words, you might not be able to find all the same things at WalMart and CostCo, or for the same prices, but if you're in the price-range of both stores and trying to optimize your purchasing, you'll still be deciding which one to shop at.

First that's a comparison for the shopper, not the retailer (and we are discussing the retailer here).

And second, you can only buy at Costco if you need the quantities they sell. Everything you can get at Costco you can also get at WalMart, but the reverse is not the case - it's not "a few extra thing" - Costco doesn't carry a huge amount of stuff you can get at WalMart.

So for the consumer buying at Costco is exclusively about what quantity they need. (Or if they have a lot of storage space + budget to store extra.) And then they also have to go to WalMart to fill in the rest.

Or in other words they complement each other - they are only partially competitors.

Costco's median customer lives in a $75K household. Poor people certainly don't have the same purchasing power to buy high-end products in bulk.

UFCW still enforces minimum wage, no benefits, and still feels like it is okay to take your money and tell you to go on strike and take a financial hit.

I worked in a UFCW store. Absolute worst mistake you can ever make. The hours are chopped so they can sidestep giving benefits, etc. There is no protection. These unions are an utter scam, selling an ideal and a myth.

I wouldn't be surprised if Walmart effectively pays better than their union competitors.