The Xbox One project was hundreds of millions to potentially billions of dollars, so I would be curious how much of that is "mostly", especially since releasing a new console quickly will hamper the earlier console's ability to maintain volume and pricing.
Unless you expect it to immediately stop making Xbox Ones, which just means Microsoft is perpetually stuck with a console that never has a chance to recoup its up front cost.
It would be a continuation of the same platform, so all marketing related costs carry over. It would be fully backward compatible so software costs mostly carry over. It would be using new hardware funded by Amd because Amd has to be doing that anyways, so no need to spend another billion dollars, just go with Amd's latest and greatest. There's no need to commission new hardware in the typical sense, nothing new to build or create. Its another xb1 revision using similar but more current hardware. I'm not saying it would be free, but it wouldn't cost anywhere near as much as launching an all new and incompatible platform. They don't need to stop making xb1's anymore than Apple has to immediately stop making iPhone 4's after they release the iPhone 5. Xb1's can continue to linger around as the cheap model until demand wanes, then it can be eliminated. Kinect would carry over as is as well.
Bog standard silicon has pretty high up front costs. Those fearsome tablets use silicon chips that must sell hundreds of thousands or millions of units to make up for the design, validation, and manufacturing costs.
If you mean creating an even more powerful APU, for example, it is a new chip and will incur much of the cost that went into Durango.
One thing to note is that AMD's little back and forth with Sony, itself, and Microsoft did mean that some of hoped-for cost savings in R&D already happened.
I'm also not sure in this scenario if you are envisioning a rapid retirement of Durango when Durango Redux comes out.
Any new hardware means incurring new up front costs. Giving Durango something over a year to pay off its up-front costs doesn't seem to be consistent with Microsoft's publically stated plans, at any rate.
Many current tablets cost mucho dinero because they are still custom designs based around Arm, hence why they have more costs. So like Apple starts with Arm then spends a chunk of time and money to customize things to their needs. I don't think MS need to go with that approach for a hardware revision. In other words they don't have to sit down with Amd and discuss what to build, instead in 3 years just go with Amd's current design be it a gcn 3.0 or whatever apu. At the 6 year time frame maybe they need to go more hardcore and spend some money for a new design, but for a 3 year revision model I don't think they need to bother. Think of it as an "Xbox One S", a refresh to keep the platform fresh and interesting, and competitive with advances on tablets, phones and ultrabooks. So rather than sticking with one box for 6 years instead they keep two in the air at a given time, a regular model and an "S" revised model. So you launch "xb1" then "xb1 s" in 3 years, then "xb2" 3 years after that, then "xb2 s" 3 years after that, etc, with only the most current two being manufactured and all fully backward compatible. 6 years is too long to sit out hardware changes in my mind, but 3 years isn't bad.
The consoles have an order of magnitude more power headroom to work with, and they have dedicated more silicon up-front. It would take multiple node transitions to match the transistor count, and with the current trends power efficiency would be nowhere near good enough to make up for the higher TDP.
If they do find a way around that problem, the problem becomes the console hardware model. With all the talk about cloud gaming and making the hardware irrelevant, it seems to already be one.
The mobile platforms will have the volumes to justify more frequent hardware transitions, or likely the smaller number of future tablet hardware platforms will have the hundreds of millions of units annually versus tens of millions of consoles over years.
I honestly think we will get to a point where it won't matter, because we will get to a point where an order of magnitude of more hardware isn't enough to really see a big visual difference. In other words I fully expect at some point that we will be able to show a game running on say an ultrabook and running on console that has an order of magnitude more power and there won't be enough visual difference there to make people think it's a big jump. Heck I'd argue we are already there for a percentage of the populace.
In any case I'm saying that they should toss out the old rigid console release model and adapt to what's happening in the marketplace and be ready to release a revised model if needed. What Apple does works well for them, they aren't always tossing everything out and starting new, they release revised "S" models that are cheaper to make but keep iPhones in the media, keeps things interesting an fresh even if they are just a hardware revision, and it lets them keep up to date hardware wise with their competition.