Nice strawman. They both go through the same unibody manufacturing process, aluminum does not cost more than polycarbonite-plastic.
There's no such thing as a "unibody manufacturing process" as applied to both aluminium AND polycarbonate.
It's possible to mould polycarbonate, where as the aluminium is machined.
Further, the cost of polycarbonate pellets is much much less than solid blocks of hi grade aluminium. The real cost in polycarbonate moulding is in the one-off cost of producing the high pressure mould.
The fact is that it DOESN'T cost more to make the MacBook Pro. Just because it's more expensive doesn't mean it's more expensive to make. The black MacBook was more expensive than the white one, does that mean it costs more to manufacture? No.