Why do you think real estate prices would crash. Most, if not all, people living here illegally are renters and are not wealthy. Why would they buy a house in a place where they may be unable to live?
Perhaps rents would decline and apartment building prices would decline, but I don’t think it would affect single family homes much.
“Nationally, 35% of undocumented immigrants are homeowners, according to a 2009 study from the Pew Hispanic Center. Of those who’ve lived in the U.S. for a decade or more, 45% own their home.”
http://www.msnbc.com/msnbc/american-dream-undocumented
Any time there would be a decrease in demand for real estate, the prices for real estate will fall.