This study found that private prison companies could help governments cut costs from between 12.46 percent and 58.61 percent. It looked like a groundbreaking study on its surface, and the private prison industry ran with it. Private prison companies were doing well enough without the academic boost. Though these companies only oversee about 8 percent of United States prisons, the number of prisoners in private facilities rose by 37 percent between 2002 and 2009, and the companies generally make a lot of money: Corrections Corporation of America (CCA), the largest private prison company in the world, has a market cap of $3.74 billion;