no one ever went broke underestimating the intelligence of...
The full saying is "No one ever went broke underestimating the intelligence of the American people/public," or some variation thereof, meaning that people being swindled won't realize or question it, which makes them a prime source of income for others. It is likely derived from the related quote typically attributed to writer H.L. Mencken: "No one in this world [...] has ever lost money by underestimating the intelligence of the great masses of the plain people." A: "We can't sell this shoddy product to people!" B: "Oh please, no one ever went broke underestimating the intelligence of the American people."
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