Costco has low prices per unit of product. But they are sold in large quantities so price per SKU is high. This is why Costco has low labor costs. A cashier scanning one average item at Costco is something like $20 in revenue, but at Walmart it is something like $5 revenue -- for the same human labor. (I remember seeing real numbers for this at one point but I can't find them.)

While a middle income person might be able to buy a month's worth of groceries at one time, and fill their freezer and pantry, lower income people often do not have the cash flow to afford making grocery trips that large. Even if the unit price is slightly higher at Walmart, they have no other option, because they can't afford to buy $50 of beef at one time if their entire shopping budget $50.