A coming Microsoft promotion will provide Windows 8 Pro to buyers of Windows 7-based PCs for just $14.99, according to my sources. The promotion begins June 2, and was first reported by my Windows Weekly cohost Mary Jo Foley.

As Mary Jo notes in her original article, Microsoft has always provided a promotion to PC buyers in the months before a new Windows version hits the street. The reasoning is simple: With a new version about to arrive, consumers might otherwise hold off on new PC purchase, scuttling sales for 3 to 6 months.

But the Windows 8 promotion is different. This time around, Microsoft is only offering two mainstream versions of the product to consumers: Windows 8 and Windows 8 Pro. And instead of offering a free upgrade to the version of Windows 8 that most closely matches the version of Windows 7 a user buys with a new PC, Microsoft will this time offering only the higher-end Windows 8 Pro.

A picture provided by Mary Jo’s sources leaves the upgrade price out, however:

win8upgradeoffer

But my sources have now told me that it will be $14.99.

The timing of the offer coincides roughly with the release of the final Windows 8 pre-release milestone, the so-called Release Preview, which Microsoft previously said was due in the first week of June. The software giant is now expected to make Windows 8 broadly available sometime in the second half of 2012.

In addition to this PC upgrade offer, Microsoft will also provide in-the-box upgrades to Windows 8 users in the form of upgrade “packs” that provide Windows 8 Pro and Windows Media Center capabilities. These upgrade packs replace the old Windows Anywhere Upgrade system and will be available through a new interface called Add Features to Windows 8. No word yet on the upgrade cost for these packs.