I think you need to read what the military have to say on this topic [1].

It is estimated that six percent of all procurement dollars are lost to fraud nation-wide annually. If you apply that amount to USAspending.gov’s figure of $700 billion dollars that the Federal Government spent on average each of the past nine years, that would total $42 billion dollars per year or $378 billion dollars lost to procurement fraud during the past nine years.

1. http://www.dodig.mil/iginformation/archives/finalwebsnips.pd...

Medicare fraud is something like $60 billion or 10% of total spending. It's conceivable that the rest of the government would approach some multiple of $100 billion, even having lower rates of fraud than Medicare.