The authors use Internal Revenue Service (the US tax authority) data, to show that overall each $1 spent on face-to-face tax audits (i.e., checking people have paid the right amount) raised $2.17.
It might cost three times as much to audit the top 0.1% as the poorest 50%, but you raise six times as much for each $1 spent.
Taking into account the large deterrent effect of being audited (those investigated tend to pay more tax for up to 14 years) ... an extra $1 spent investigating the top 10% brings in a whopping $12.
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