An estimated $2.6 Billion is illegally gambled on offshore gambling websites by residents of the United States each year. By comparison, between $2 to $3 Billion is wagered on sports in legal casinos in the state of Nevada.
In total, Americans place up to $500 Million in bets, both legal and illegal, on sports each year.
(Additional information about illegal online gambling.)
Source: Geoff Freeman, “Policymakers must get on board with online gambling,” Las Vegas Review Journal, September 19, 2103.