Per a Walter Pincus story in the Washington Post, "as the United States withdraws its combat forces from Iraq, the government is hiring more private guards to protect U.S. installations at a cost that could near $1 billion, according to the special inspector general for Iraq reconstruction. "
For installations, read bases (which aren't permanent, of course). Also from that article:
The Central Command study found that of the armed private security personnel working in June, 623 were Americans, 1,029 were Iraqis and 11,580 were third-country nationals. Most of that group "were from countries such as Uganda and Kenya," according to the inspector general's report.
These aren't mercenaries but "contractors." The US is in Iraq not for the oil but to spread democracy.