Microsoft Volume Licensing is not itself a factor in big price reductions, it's a sliver less expensive than boxed retail products at MSRP. You have your licenses available online via eOpen, and you get 1x shrink wrapped bundle of CDs for all your products ordered...1x CD per product name.
It is still purchased through a "Microsoft partner near you"/authorized reseller. (such as me

)
Most of our clients have their MS products through us via VL.
The price savings for Gov't is pretty decent. Microsoft actually does have great pricing for certain categories. On the extreme end...take a look at the deep discounts they give to organizations that quality for Microsoft price slashes through Techsoup.org.
Through Techsoup I have setup some clients of mine with software that would have cost them, well...say for a network with 3x servers and 50 workstations..I'll set them up with server operating systems for 3, CALs for 50, XP Pro licenses for 50, Office 2K7 Enterprise Edition for 50...they'll pay just several hundred dollars instead of 15 grand!
Example..
http://www.techsoup.org/stock/category. ... +MS&Page=6
Microsoft Small Business Server 2009 Premium Edition...just $62.00
Instead of $1,899.00
http://www.microsoft.com/sbs/en/us/pricing.aspx
Windows XP Pro..8 dollars!
Office 2K7 Ent Edition..something like 35 dollars per..instead of 499 or more.
Their government discounts are not that deep..but they vary a lot, according to the size, type, etc. So quotes are often custom made by us resellers via our Microsoft rep.
Gov't agencies also usually go onboard with a special pricing plan called "Software Assurance"...so that they can spread out of the cost of the goods over a 3-5 year plan...and they quality for free upgrades to any newer version of product they are licensed for that Microsoft releases during the period that their SA license is valid for.
The other unforeseen costs...most offices are running on line of business software that is designed to run on Windows. Existing databases, accounting, management, ...tons of software. As well as the peripherals such as printers, AIW machines, faxing, etc. The cost to change those over to machines that are supported by *nix...and the cost of migrating existing data into their new *nix based software..that's pretty big. The period of segway would take quite a few years to recoop the cost.
And the cost of support for these computers...servers, desktop computers. Consultants that support *nix machines are pricier..since they are far and few. They're often up to 50% higher hourly rates than us Windows guys. I'm $125.00 per hour as a Windows SMB consultant. A good *nix consultant would be a bit more..and that would add substantially to the above time period to recoop the cost.