From Factcheck.org
"30% percent of all Federal Prison inmates are illegal aliens."
Both of these claims can be traced back to that same April 1, 2006, episode of "Lou Dobbs Tonight" on CNN, in the same segment, with the same correspondent, Christine Romans. But the e-mail misrepresents what Romans said. She gave figures for people who are "not U.S. citizens," a category that would include legal residents as well as "illegal aliens."
Romans said that "according to the Federal Bureau of Prisons, 30 percent of federal prisoners are not U.S. citizens," adding that "most are thought to be illegal aliens." Actually, the Federal Bureau of Prisons does not keep figures on illegal immigrants. What solid numbers we can find point to a much smaller figure. A Department of Justice report from 2003 found that only 1.6 percent of the state and federal prison populations was under Immigration and Customs Enforcement jurisdiction, and thus known to be illegal immigrants. Half of these prisoners were detained only because they were here illegally, not for other crimes.
The Bureau of Prisons does track prisoners by offense when information is available. By that metric, 10.7 percent of prisoners in federal jails were incarcerated for immigration offenses in 2009. In 2006, when Romans gave her report, the figure was 10.2 percent.
The "$3 million dollar a day" figure is based on the false assumption that 30 percent of all inmates are illegal immigrants, and thus is greatly inflated.