"In 2001, the Social Security Administration concluded that undocumented immigrants "account for a major portion of the billions of dollars paid into social security that don’t match SSA records," which payees, many of whom are undocumented immigrants, can never draw upon. As of July 2003, these payments totaled $421 billion."
$421 billion dollars paid into a system they can't touch.
More on the myths about illegal immigration and the dangers of the raids is discussed over at Digby's.
(edited for bizarre grammar and incoherence.)