Actually, they don't get such savings. "Buying in bulk" is a myth. If it costs us $50/8-GB, then adding that memory to the console will jack up its price by $50. At least.
Let me prove it to you. On NewEgg right this second, you can get a single 8-GB DIMM for your desktop computer for $49.99. On the bulk spot market, the 4-gigabit chips themselves cost $3.11/chip when buying in very large quantities of 10,000 chips at a time. Since you need 16 of those chips to create a DIMM, that comes out to $49.76 per 8-GB.
In other words, within the margin of error, the price for a single DIMM on NewEgg roughly equals the bulk price on the spot market.