The 8-core Zen 2 chiplet is around 70mm^2. The 330mm^2 Vega 20 might be a good start for a 13 TFLOPs GPU, but a 13 TFLOPs Navi could be significantly smaller. Navi could be designed to clock higher, and/or to have significantly more ALUs per CU (more than 64).
See for example the RV670 -> RV770 transition. AMD increased the ALU and TMU amount by 2.5 times while increasing the chip size by around 33% in the same 55nm process. Considering Turing has separate FP32 and INT32 ALUs, nvidia too increased significantly the number ot ALUs from Pascal to Volta.
While I doubt we'll see a 13 TFLOPs capable GPU measuring below say 200mm^2, it might be significantly smaller than Vega 20's 330mm^2.
We know both Liverpool and Durango were about 350mm^2 big, and that's where I'd point next gen SoCs to be. If we subtract 70mm^2 for CPU cores (+L2 +L3), we'd get around 280mm^2 worth for the rest of the SoC (GPU, memory controller, I/O). It's not out of the realm of possibilities to have a 280mm^2 GPU capable of hitting 12-13 TFLOPs (4 engines, 64 CUs, 64 ROPs at 1.6GHz).
Cost for such a SoC would probably depend on whether they're using EUV or DUV. The former should be fairly expensive, probably $150-200 (considering the 350mm^2 SoCs on 28nm back in 2013 were ~$100). EUV should be cheaper, and with the recent news of TSMC going with mass production EUV in March.
Memory would be a problem, because despite the recent downward trend the prices are still very bloated from over 2 years of "abuse".
I too would point to $200-250 for 24GB GDDR6 during H2 2019.
So we're at $150 for SoC and $250 for memory. Assuming everything else
similar to the PS4, we'd be looking at close to $600 for BOM + manufacturing (without that rumored WiiU-like controller).
Digikey is putting 2K units of GDDR6 chips at
around $20/chip at the moment.
I have no doubt Sony and Microsoft can get much better deals for tens of millions of chips, but aren't those $6/GB price you're seeing for DDR4? GDDR6 is an entirely different beast.
Moreover, most rumors point to a March 2020 release date for the PS5. That's 12 months from now, but volume production for the console (which is when the first memory chips would be ordered) would need to start some 4-6 months earlier, so 8 months from now is the price that matters.
Unless Digikey is charging ridiculous margins, I don't see how GDDR6 is going to reach $5/GB in 8 months even if bought directly from the manufacturers.
According to
Buildzoid (at ~1m30s) GDDR6's layout demands make it very difficult and/or ridiculously expensive to create a 512bit bus. Honestly, nvidia's TU102 PCB show that even a 384bit bus on GDDR6 seems super complex and maybe too expensive to put into a home console that is capped at $500.
If Sony or Microsoft want a total bandwidth too much above 512GB/s, then they might be better off using HBM, but then they can't go with insane amounts like 24GB.
In the end, I think 16GB GDDR6 at 256bit, plus around 8GB 64/128bit DDR4 exclusive for the CPU, would be the most plausible scenario at the moment.
