Yes, the off-the-shelf package includes a large fee for Microsoft support, and the right to move the OS from one machine to another. You don't get either of those with the OEM versions.

One obvious factor in the fall of Windows pricing is the fall of PC prices. Microsoft could charge a lot when PCs cost $/€/£2,500 each.

As PC prices fell through $/€/£1,500 each and $/€/£500 each to today's levels, the cost of Windows would have been an increasingly large proportion of the total. That was bound to be unsustainable, so I imagine (without any proof) that the price of Windows has come down much like the cost of hard drives and screens....