We have an estimate!
Fraud typically gobbles up around 7% of all big contracts.
This is from Watchdog Over Stimulus Spending Toes a Delicate Line, by Neil King, Jr., WSJ 3/9/09. Altho no source is cited, the implication is that this came from Earl Devaney, who according to the article heads the federal Recovery Act Accountability and Transparency Board.
I guess that’s just fraud. Waste and inefficiency are things entirely different.