I realize it may seem like Microsoft has the IRS over a barrel, but keep in mind that the US Government does have a bit of negotiating power on prices due to its sheer size and the common use of large comprehensive contracts. I'm sure they aren't paying the average price for support.
Analysts noted earlier this year that Microsoft had dramatically raised prices for Custom Support, which previously had been capped at $200,000 per customer for the first year. Instead, Microsoft negotiates each contract separately, asking for an average of $200 per PC for the first year of Custom Support.
So they are paying $30 million to migrate to Windows 7. According to the article they have 58,000 PCs left to upgrade. That's $517.24 per machine. Licenses for Windows 7 Ultimate and Pro seem to be between $150 and $200 a pop on GSA Advantage. Throw in some CALs, software assurance, or other essential software, and it is probably leaving less than the $200 per machine average for the Custom Support.