A Family Dollar store in Queens. By weight, items can actually work out more expensive than Walmart or Costco, for instance.

While online retailers have transformed the landscape of American commerce, the largest three dollar-store chains are prospering offline, opening more than 1,800 stores last year.

The cost of a trip can be so negligible – the average customer drops $29 a month – and dollar stores have grown so ubiquitous, that it’s hard to countenance what economists confirm: visitors to dollar stores are often paying more than well-off consumers who shop elsewhere.

“If you’re budget-constrained, then you make choices that are not optimal,” said the professor John Strong, a dollar-store expert at the College of William & Mary.

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The bags of flour at a Dollar Store just south of San Francisco cost only $1, but they also only weigh two pounds. Most bags in the supermarket are five pounds, and can be scored for less than $2.50 at cavernous retailers like Walmart or Costco – though these require time and, often, a car to access. The dollar-store shopper would pay an equivalent of $5 for this much flour.

Dollar store raisins are only 4.5 ounces. At a big box store, however, 72 ounces of raisins cost $10.50 – meaning dollar store customers are paying 52 percent more.

Cartons of milk at a dollar store are only 16 ounces – which prorates to $8 per gallon, more than what you’d pay for even top-of-the line milk at Whole Foods.

Deep-discount retailers have flourished in recent decades, popping up like mushrooms in the depressed locales big-box stores economically eviscerated in the decades before.

The Family Dollar chain opened up…