They can offer a $100 discount if you trade in your PS4, and then actually make a profit by selling it used.
Seriously, they will be able to reduce price in line with manufacturing improvements. Those aren't any different than what MS/Sony can/will do,
and in fact the proportional price differential will almost certainly reduce, as plenty of components just aren't as amenable to manufacturing process improvements (e.g. power supply).
Especially compared to PS4 which lacks Wii U's tablet nor MS' Kinect: it's basically just CPU/GPU/memory and hard drive and power supply,
so proportionately more of the production cost is amenable to process improvements reducing cost.
Sony buying a memory producer (which has and is producing for Wii U) will let them get at-cost memory.