Again, Microsoft isn't paying for the cost of the materials or the labor, they're simply paying what the vendor is charging, and there is no rule anywhere that says that the vendor has to charge over their costs. To Microsoft, the hard drive is just a PART. You're WAY overanalyzing the situation.
But, to answer your question ...
The cost of the raw materials drops just like anything else in a free-market economy; Hard Drive makers don't make every last thing in the drive, they buy parts from suppliers just like anyone else. Labor goes down as the Hard Drive makers replace humans with machines, the cost of the clean rooms and equipment is amortized over the cost of each manufacturing run, and technological advancements reduce costs across the board.
None of that is even close to relevant in the Microsoft situation, though, since 8GB hard drives aren't made anymore, and 10GB drives (what Microsoft switched to) are quickly being phased out too. When that happens, the price will drop even further as the hard drive suppliers clears out obsolete inventory.
Seriously, you might want to rethink acting like you have an MBA around here. You'll get eaten alive.