Getting them to leave is a cheap, easy, two step process. Step one, impose fines on any business that employs illegals. Step two, cut off all government benefits for illegals - saving the government money.
Denied any source of income they evict themselves.
Laws are already on the books...they simply need to be enforced. You don't have to 'round' anybody up. Just hit their employers. No jobs and they leave. And it wouldn't take long...a couple of company presidents being perp walked for employing illegals will result in a huge number of unemployed illegals.
http://www.csmonitor.com/2006/0706/p09s01-coop.html
How Eisenhower solved illegal border crossings from Mexico