10% of entire prison population are immigrants eligible for deportation
According to Julie Myers, head of Immigration and Customs Enforcement ("ICE"), around 10% of the total U.S. prison population are immigrants who could be deported (link). In order to avoid confusion (see the David Leonhardt/New York Times smear of Lou Dobbs), it's important to note that that doesn't just include federal prisons but all prisons and jails at the federal, state and local level. And, it also includes persons in various immigration statuses who are eligible for deportation, not just those who were illegal aliens at the time of committing the crime.
Myers presented her plans to work to deport those criminals to the House Appropriations Subcommittee on Homeland Security, and estimated that the cost could be over $2 billion per year. Her plan involves information sharing between various agencies.
Both Reps. David Price (D-NC) and Harold Rogers (R-KY) seem to support deporting the criminals, with the former raising questions about the efficacy of her plan and the latter saying just that the plan needs work. The fact that a Democrat is raising questions raises questions of its own, but on the other hand the plan is coming from Myers so there probably are some issues with it.