The amount illegally gambled on National Football League games is estimated to be between $80 Billion to $380 Billion.

From chicagosports.com (part of the Chicago Tribue):

After all, Nevada casinos won $173 million on football bets last year. Some $94.5 million was wagered legally on the February 2006 Super Bowl alone. Illegal NFL gambling reportedly nets between $80 billion and $380 billion annually. The league does its best to publicly distance itself from the unsightly underbelly of organized gambling, but the game owes part of its unquestioned popularity to the weekly spread.