Worst case scenario the CPU portion will be Bristol Ridge with 4 or 8 Excavator cores. Best case scenario will be Raven Ridge with 4 or 8 Ryzen cores.
You need to take into account that Intel core i series chips are all WAY overpriced, especially the top tier models. This is because Intel has had no competition for years. Now that Ryzen will go toe to toe, Intel will have no choice but to drop their prices drastically.
For this reason AMD will have to sell Ryzen at a very reasonable price to make sure consumers see the performance and price advantage and side with AMD. There are a lot of people out there right now saying "thanks AMD, now we will be able to keep buying Intel, just at a much lower, and more reasonable price, all thanks to you!". AMD fans will buy Ryzen regardless, but it will take time for the public to get it through their heads that Intel isn't the only option, and that AMD Ryzen is the better deal (barring any unforeseen complications). A much lower price than the Intel equivalent will accelerate this transition.
This whole situation makes Ryzen cheaper, right off the bat, and when you add in an ongoing, important customer, like MS, who will be buying the same chip in VERY large quantities, you can expect the cost of Ryzen (as well as Vega) to be at an acceptable cost for a Premium console late 2017.
You also have to take PS5 into account. We all know its coming, late 2018 or 2019, and it will no doubt have Ryzen if they stick with x86, which is most likely, since AMD has said they are working on future APU's for MS and PS. For Scorpio to be a year or two "behind", with an Excavator CPU, PS5 will end up being spec'd so much higher that it'll leave Scorpio worse off than XB1 was in comparison to PS4. As long as MS sticks to "we aren't coming out with a console every couple of years" than Scorpio would be left in a bad position without Ryzen cores in it once PS5 launches. There would be at least 2 or 3 years, if not more, that Scorpio was well behind PS5 before its successor showed up.
Vega also makes sense with a Ryzen CPU, because Ryzen is a big leap and a new x86 design for AMD, and will be the new standard going forward. The Radeon GCN cores have had a fairly similar design for years now, but Vega will make a big leap with its new NCU design, which will also be a new standard going forward. It makes sense for MS to jump on board now and have the same standards in Scorpio that PS5 will have, so it's not left behind next gen.
All these reasons (there's a lot more) are reason enough for MS to make the APU Ryzen+Vega. Even if it means they have to subsidize Scorpio to some degree early on in its life cycle.
