Just limiting deductions for the wealthy

This I am pulling from Greg Mankiw, with the actual source being the White House blog, namely Sperling and Furman:

Bottom line: If you apply a $25,000 deduction cap only to households with income above $250K, phase in the cap gradually as income rises above $250K, and exclude charitable giving from the cap, you increase revenue by only $450 billion over ten years.