Many prisons in the United States are already privatized. This creates a issue because it is in the corporation operating the prison's best interest to keep their cots full since they get paid per head. To that end they lobby for laws that supply more heads for them to charge the customer (the government) for. For example: http://colorlines.com/archives/2010/10/arizonas_draconian_and_constitutionally_suspect.html
Sure, they might be able to house the same number of prisoners for less money, but if they are using their profits to "reinvest" by turning more people into criminals, than that is a problem. In the end, it costs the taxpayer and the economy more because we've moved yet another productive member of society into a position of dependence on the government. The fact (yes, this is a fact) that the "land of the free" has the highest per capita incarceration rate of any country in the world disgusts me. For more information see: http://en.wikipedia.org/wiki/United_States_incarceration_rate
I propose we eliminate federal prisons altogether. Keeping people locked up in cages is just a waste of resources. Instead, we should use alternate forms of punishment. Fines, wage garnishments, etc. For henious crimes, just use a quick capital punishment and get it over with.