Don Luskin makes some calls on Krugman's sourcing and does taxes better than I.
At most, Bush's tax cuts would reduce a 28% tax on $2000 to a 15% tax -- for a savings of $260, only about a third of the $725 savings in the example.
My calculation was apparently more generous to Krugman than it should have been. It happens.
Luskin has more, read it all.