AMD is not in the same position that they were in last time around. Previously, they were desperate for ANY degree above COGS. Now, anything that they do in semi custom is essentially an opportunity investment. They could put more resources towards their own branded devices, or they could put them towards MS's chip. Each has an expected return on investment. MS has to offer them enough to not only cover their costs, but to also beat the return on doing anything else.
This is why I suggest that a single CCD Medusa Halo would be a good fit. It's a completed design, so there's no new R&D to pay for. Unused IP in the chip can be fused off, reducing licensing costs. So, pay the smaller integration cost up front and buy the rest on a volume discount.