We're losing hideous amounts of money on refundable tax credits like the Earned Income Tax Credit, the Child Tax Credit, and the American Opportunity Tax Credit that. For the most recent data year available we lost $18 billion on the Earned Income Tax Credit. According to the Treasury Inspector General for Tax Administration the Office for Management & Budget granted special exemption for a couple programs to stay off the main Improper Payments Elimination and Recovery Act of 2010 reports for the time being. All in all we're losing around $30-50 billion just from your averge Joe Sixpack.
If we repealed most of these refundable tax credits the amount of potential fraud would greatly reduce since such a vast number attempt fraud relative to those credits alone. That would free up around two thousand personnel that could be transferred to other audit functions. From the outside it doesn't seem like it but you should never discount how big fraud by so many little people can be in the aggregate.