In order to gain 99% of the network hash rate, you need to increase the current rate by a factor of 100, and you would need enough mining equipment to generate 10,000,000,000 GH/s. At $2 per GH/s, that would cost you $20 billion.
The U.S. government has the ability to spend that much money, but keep in mind that the cost of such a project would be twice the cost of the Defense Department's largest program. Approval for such a project will never happen unless Bitcoin is perceived by many as a major threat to the U.S.
That is true if you want to increase the current rate by a factor of 100 in a couple of days or months. But if you look at my scenario and think about it a little, you can see that they don't need to hurry with the attack too much. They can start small and fund next stages of the expansion of their hashing capacity from the profits made by the previous stages. They could start with as little as 25% of the current hashing capacity (or, maybe even 10%). Given the current exchange rates that would give them %45000 per hour of income at the beginning which would all spend on more hashers and electricity. They could even repay the "loan" taken from the taxpayer's money well before they reach into 50% of the hashing capacity. And the more they conquer, the more income they will get from it.
You know, the point of the scenario is that nothing in the network is going to indicate that something bad is going to happen ... until they pull the plug. They could even recoup the price of the hardware (by selling the generated coins that are abundantly flowing out of their 99% of hashing power) they used to pull out the attack before they pull the plug.