Going off a typical console launch price..
£500 with an OS and controls...
That would piss over a console, sticking that lot in a closed box would be tricky given heat and power requirements but chuck in bulk orders and other big savings and it would work out quite a bit cheaper to build then the price above.
The sheer savings in PCB components alone is quite large when moving to a single PCB in a console.
Now all you have to do is figure out how to cut some more of the cost off of that.
Figure out how to get it into a box significantly smaller.
How to cool all of that well enough that it doesn't end up being significantly louder than the launch X360 in the small box meant to fit in a home entertainment cabinet. Hint: we can't use 200 mm exhaust fans, we can't use 120 mm or even 92 mm CPU cooling fans unless that fan is also used to cool the entire system, etc...
All of which means expensive and/or exotic cooling solutions driving the cost up quite significantly.
That doesn't even factor in the cost of attempting to put all of that on a single PCB that will fit in a case similiar to the size of the original X360.
Speaking of a case the size of the original X360, that means no 3.5" HDD. So you can roughly double the cost of the included 2.5" HDD. Or triple to quadruple the cost for a 7200 RPM drive such that loading times and/or streaming times won't be absolutely atrocious on games capable of utilizing 4 GB of RAM.
Next gen consoles are going to be a huge balancing act when it comes to cost, performance, power consumption, cooling and noise.
I'd say the most powerful practical GPU you could stick in one would be a 7750 due to the low power (75 watts or less at load) and decent performance at that level. Move up to a pitcairn and suddenly the console is going to be dangerously close to consuming over 200 watts at load for the whole system.
And yes, for a console going into a home entertainment system, the manufacturer's are going to want to try to hit 200 watts or less, although I have a feeling we may end up bumping just north of that with quieter and cooler versions coming out after a year or two.
If you want to try to imagine what in the PC landscape could end up in next gen consoles, the best place to start is with laptops and laptop components.
All of the above is also why it's incredibly unlikely that either company is going to even remotely try to enable 4k gaming. If we're lucky, 1080p will become standard, with high profile 60 FPS games likely sticking to 720p.
Regards,
SB