Business Insider hired an analytics firm named Numerator to create a report that finds on the average Costco customer [0]. I couldn't find the original data or report in Business Insider's summary, but from its article:

"Numerator found that Costco's typical shopper in the US is an Asian American woman between 35 and 44 years old who is married and living in a city in the Pacific Northwest. (Costco was founded in Seattle and is based in Washington.) She typically has a four-year degree or higher level of education and earns more than $125,000 a year."

I'm not sure whether the income of the "typical shopper" was defined based on a median or mode, so there could well be a large variance of customers with a lower socioeconomic status. A separate article by Investopedia [1] also asserts that Costco mostly opens stores in affluent suburban areas, and originally targeted college-educated and financially wealthier customers.

So, Costco's customers are likely skewed to have a higher socioeconomic status, based on the location of most of its stores in wealthier suburban areas and findings from an analytics report (with some skepticism towards the article about the report, as the raw data and methods weren't shared, as far as I could see).

[0] https://www.businessinsider.com/typical-costco-shopper-demog...

[1] https://www.investopedia.com/articles/insights/061516/whats-...