According to a study by economist Edgar Feige, up to $2 Trillion was earned by workers in the United States in 2012 and not reported to the Internal Revenue Service (IRS). This figure is mostly from tens of millions of Americans who earned money as nannies, barbers, and web designers who got paid under the table.
The IRS estimated in 19992 that the Federal Government was losing $80 Billion a year to undeclared earnings. In 2006, the IRS estimate of lost tax revenue was $385 Billion.
In total, people in the United States hold as much as $750 Billion in cash.
Source: James Surowiecki, “The Underground Recovery,” New Yorker, April 29, 2013.