Many of the illegal immigrants are paying taxes, but not getting benefits.
The total is in the billions. The IRS even provides an alternative to the Social Security Number so that it can collect taxes from foreign workers.
Illegal workers are subject to withholding taxes. Many end up paying more than they owe.

A proper registration system and route to citizenship is necessary to clean up the contradictions in policies and let these productive workers apply.

See Associated Press article, msnbc, "At tax time, illegal immigrants are paying too.":
http://www.msnbc.msn.com/id/24054024/

Proposals to regularize the system are dead for this session of Congress. Lies, many by omission, from widely audited personalities who are interested only in their ratings, have killed them - Lou Dobbs rants on CNN showed his propensity to make untrue statements that would shock rather than inform.
He may have moderated his views since being separated from CNN, but I would trust him no farther than I could throw a mature crocodile.