"Our analysis indicates that delays in raising the debt limit in 2011 led to increased borrowing costs on certain securities," the GAO report says. "We found that the 2011 debt limit event led to a premium on Treasury securities with maturities of 2 years or more while Treasury securities with shorter maturities either experienced no change or became slightly less costly relative to private securities. Applying the relevant increase or decrease in the yield spread ... to all Treasury bills, notes, bonds, CM bills, and TIPS issued during the 2011 debt limit event period, we estimated that borrowing costs increased by about $1.3 billion in fiscal year 2011."
But these costs aren't contained to FY 2011.
As GAO notes, "Many of the Treasury securities issued during the 2011 debt limit event period will remain outstanding for years to come. Accordingly, the multiyear increase in borrowing costs arising from the event is greater than the additional borrowing costs during fiscal year 2011 alone."
The report does not account for the debt limit fight's negative impact on consumer confidence and credit, which harmed and delayed the economic recovery.
You can read the full report here (PDF).