Yahoo finance says roughly 10% of prisons are privately operated. One company has capacity for over 120,000 inmates, so I'd estimate the private prison population is in the low to middle 100ks.
A huge segment of policing, courts, and prisons is dependent on the Drug War, and there is enormous public and private sector profiteering in it. But I'd say the other bad effects, like the proliferation of SWAT, no-knock raids, and militarization are worse than the expense and corruption.
Some of the other answers here are low due to counting only federal inmates, or being out of date.
"The trend toward privately operated correctional facilities has continued with 85,604 adults (3.7% of the total US prison population) now housed in 107 privately operated prisons as of 2011[13] Companies operating such facilities include the Corrections Corporation of America, the GEO Group, Inc. (formerly known as Wackenhut Securities), and Community Education Centers. In the past two decades CCA has seen its profits increase by more than 500 percent.[14] The prison industry as a whole took in over $5 billion in revenue in 2011.[15]
"According to journalist Matt Taibbi, Wall Street banks took notice of this big influx of cash, and are now some of the prison industry's biggest investors. Wells Fargo has around 100 million invested in GEO Group and 6 million in CCA. Other major investors include Bank of America, Fidelity Investments, General Electric and The Vanguard Group. CCA's share price went from a dollar in 2000 to $34.34 in 2013.[15]"
Distinctions between corporations and government aside (their interests are the same, to a first approximation; and of course corporations are government-created entities)... the "drug war" is an effective pretext for social control, particularly in the top jailer of its own people: the US. Puts certain demographics in a horrific box with no freedom.
ProPublica puts the percentage of inmates in private facilities at around 8% in 2010. The 3.7% figure appears to come from a cited academic work that I can't verify, so I'm not sure how much stock I'd put in that figure. ProPublica tends to put out pretty reliable stats.
More concerning than the total percentage is the rate at which this number is increasing (~40% per year).
edit: so that question was interesting enough for 3 people to look up the answer at the same time ;) Also I had to double check, wtf at the 100,000 prisoners being only 4% of total US prisoners..
