There's a flaw in your logic. Windows OEMs usually have a 20-30% profit margin in prebuilt PCs, Laptops. But the parts suppliers often have 60-100% profit margins. So you can't use retail pricing of each of those parts.
MS is going to be buying the Magnus APUs from AMD in bulk then provide them to OEMs at cost. That was according to
Proelite
OEMs will take in their 20-25% profit margins. MS isn't seeking profits from hardware, their goal is the proliferation of the open ecosystem, to attach Xbox hardware to the larger overall PC Gaming hardware ecosystem.