Illegal immigrants are responsible for roughly 20 percent of the $2 billion in unreimbursed care that Southern California hospitals deliver each year. Even if you consider that factor, you have to conclude that it's the larger problem of just simply having so many uninsured patients that is a key driver of rising hospital costs.
In order to receive federal Medicare and Medicaid payments, a hospital must agree to treat and stabilize everybody who shows up to a hospital ER regardless of their ability to pay or their immigration status. That means undocumented immigrants who show up at the emergency room will receive treatment regardless of their immigration status or whether they're insured. But so will legal immigrants, naturalized citizens and native-born Americans.
http://www.hasc.org/lott.cfm?ID=73623