The cost of servicing government debt is considerably less than the payments made by the government, because to a large extent we're making those payments to ourselves. This is true even if all of the debt is held by foreigners. In that case, you can view interest payments on the debt as a fee we pay to foreigners for allowing us to avoid raising taxes, which means we can (if we choose) maintain higher bank balances and earn more interest. Via that somewhat circuitous route, the debt payments still come right back to us. So it might have been more accurate for Krugman to say not that the service on $1 trillion in stimulus spending will cost us .13 percent of GDP, but rather that it will cost us zero percent of GDP. If he'd done that, though, it might have been more obvious to readers that he was pulling the wool over their eyes by ignoring the true costs of spending.