I would think that a large OEM, with all the supply chain advantages that are implied, could easily compete against a Microsoft device.
Think about Samsung, for example. They have factories build RAM, displays, etc, that - by your logic - could be almost free to the device building division. Microsoft has to BUY all that stuff from Samsung.
My point is that Microsoft has already said that the Surfaces prices will be "competitive" with the iPad. If you believe that Samsung's cost to build a tablet is comparable to Apple, then how can Samsung sell their tablets at a "competitive" prices as Apple, given that they have to pay Microsoft $99 for the OS.
All this race to the bottom will drive the Samsungs out of the market, leaving only Microsoft as the monopoly hardware maker. In a monopoly, consumers lose.