Here's a thing.

Up to the end of January, UK Retailer PC World/Currys were selling the boxed version of Windows 8 Pro upgrades at a penny short of 45 (online) and 50 (in-store).

Sometime soon after February started, the online price stayed at 45, but the in-store price went up to the Microsoft price of 190.

However, today I notice that the in-store price has come back down again to 50.

That's a pretty big price fluctuation up and back down again to happen within a week.