From the Washington Post: 

“In a research note Tuesday, J.P. Morgan analysts estimated that federal furloughs will reduce national income by a total of $1.3 billion per week. As a result, the shutdown could shave 0.12 percent off fourth quarter GDP growth for each week it goes on. That forecast doesn’t account for any knock-on effects on the private sector or dent in economic confidence, which are harder to quantify. All that lost income could be recouped if Congress later agrees to give those 770,000 furloughed federal workers back pay. But for now, that’s very much uncertain. Republicans in Congress are split on whether to agree to retroactive pay to workers who get furloughed.”