I've noticed that APU tends to hover around $100, though that might change with XBOX, then again maybe not.
They could further save money by going with a internal PSU ($6.00 saved), that would generate more heat, ok so vapor chamber cooling.
The system would run a lot more silent (my laptop uses it), secondly they could run the memory clock speeds much higher due to more efficient cooling.
Memory cost hasn't been dropping so I'm expecting an increase to the bill-of-materials there, which is the main reason I think it will be 16GB GDDR6.
The rest doesn't really change as they are basic components, it's the memory, UHD/no UHD and the APU itself that will make the BOM fluctuate.
As I've said before I'm pretty sure both companies are taking a hit on the low SKU's and off-setting that through services and/or the higher-end SKU.
2019 seems troublesome for them to launch, you'd first probably need AMD to have Navi out of the gate in the form of desktop GPU's.
I've yet to see a console launch with a architecture before it's desktop counterpart, so 2019 seems 50/50 in that regard, lower even if Navi is further delayed.
If they go with a full fledged Vega that gives us 64CU (plus maybe 6 added CU's?, so 70), plus the power reduction itself from 7nm (thus less heat generated).
You'd have 70 (compute unite) * 64 (shader/stream) = 4480 (stream processors) * 2x 1340Mhz (instructions per clock) = 12TF, with a vapor chamber setup.
60% power reduction compared to 14nm, Scorpio runs at 1172MHz on 14nm, while 60% doesn't equal the amount of heat reduction they could still bump clocks to 1.3Ghz+ I reckon.