In the final audit report by the Office of the Special Inspector General for Iraq Reconstruction Funds the US government mentioned many accounting errors that mean the precise amount of wasted taxpayer money will probably never be known.
Billing documents by contractors were not reviewed thoroughly (or at all) by the US contracting officers, leading to such costs as a $1.41 pipe fitting costing taxpayers $80. In an earlier report the Inspector General mentioned the contracting officers themselves dealing with personnel shortages and changing goals were under pressure to show results quickly. In one case only one contracting officer was available to review $2.5 million contract with Dynacorp for police training; after review the government recovered $60 million in wasted or fraudulent spending.
These investigations not only uncovered sizable waste, but also resulted in 87 indictments, 71 convictions, and $176 million in penalties for crimes such as bribery, bid-rigging, embezzlement, and theft.
If you are interested in reading the entire report, it can be found here.