COSTCO AFFLUENCE: You might be surprised to know that 54% of CostCo's Wholesale Club (CWC) members are considered wealthy, or "affluent", with only 15% just "getting by" or "poor".

BUSINESS VS CONSUMER: Approximately 24% businesses, 76% individual consumers. Even though business customers drive about 60% of CostCo's revenue, about half of that is for home use, so it's about 30% business revenue and 70% individual consumer revenue.

HOUSEHOLD: 35% of CWC members have 2 person households. 56% have three or more in their home.

I have a ton of paid tools at my disposal for market research. For much more detail on other demographic data for CWC, along with info from BJs and other wholesale clubs, set up a call with me on Clarity.

Think Al Bundy. Use him as your demographic, you will not go to far off track. No insults intended, but you need to have a physical character that represents your target audience. If you can't find one, make one up. But you can usually find one on TV.
Don't stop taking massive action.
Best of Luck,
Michael T. Irvin
michaelirvin.net
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My extended family and most of their neighbors are Costco shoppers. So, while I cannot point to statistical studies, I can speak from first-hand "annecdotal" experience.

Yes, from what I've seen, Costco shoppers tend to range from middle class all the way up to the very wealthy, believe it or not, depending on region and culture.

The region I'd point to in this case is the state of Utah, including both LDS and non-LDS individuals. Households would often be affluent. Professions include doctors, dentists, senior engineers, business owners, heads of hospitals. Education would usually be a bachelor's degree but including many MDs and PhDs.

Often the women do the shopping, but some of the female Costco shoppers I know personally have advanced degrees and high paying full-time jobs. Men also shop at Costco -- especially when preparing for large events that feed multiple families. Typically Costco is not the only place people shop -- but the most time-efficient. Cost is actually not the primary motivation, from what I've seen. Time is. People are seeking a single, simpler bulk solution.

"Families" is the most important word here, I suspect, when defining the Costco demographic. At one point years ago, I inherited a Costco member card; but I found it unwieldy to shop for just 1-3 people there. Generally, from what I've seen, Costco shoppers represent families with at least 2 children over the age of 8.

For what it's worth, that's my unscientific answer.