True Cost of Government Shutdowns, Staff Report, Permanent Subcommittee on Investigations, Committee on Homeland Security and Governmental Affairs, United States Senate, One Hundred Sixteenth Congress [open pdf - 2MB]
From the Executive Summary: "The Constitution grants Congress the power of the purse. Specifically, Congress has the sole responsibility to raise revenue and appropriate money to fund the federal government. When Congress and the President fail to reach agreement on legislation to appropriate funds, affected federal agencies shut down and suspend most activities. During this time, federal law prevents most federal employees from working and most are sent home without pay. In recent shutdowns, however, the spending legislation ultimately agreed upon by Congress and the President provided back pay for federal employees. As such, the American taxpayer funded furloughed federal employees' salaries for the duration of each shutdown, even when employees were not permitted to go to work. This report documents the cost to the American taxpayer of the last three government shutdowns."
Senate Committee on Homeland Security and Governmental Affairs: https://www.hsgac.senate.gov/