Inflation adjusted this a ~15% price drop from the initial cost of Vista. But, the real shocker is the preorder price of Windows 7 Home Premium ($49.99) and Windows 7 Professional ($99.99).
Frankly the boxed OS prices for Windows are moot.
Individuals get the new OS license with the new computer and businesses go through resellers. 7 isn't any 'cheaper' than Vista for any significant number of Microsoft's customers.
Honest question though: How many people do you know who still build desktops? It seems everyone I know has gone laptop + external monitors in the past few years. (with hand-built 'server' machines all running linux)