We all know a 128-bit 4800-6400MHz(76.8-102.4GB/s) DDR5 won't be enough.
For comparison:
RX 6400 has 3.6TFLOPs, 128GB/s GDDR6 + 16MB IF.
RX 6500XT has 5.8TFLOPs, 144GB/s GDDR6 + 16MB IF.
It has 61% more TFLOPs, yet It performs only 28% better(
Link) in Full HD.
6 WGP IGP at only 2GHz has already 6.1 TFLOPs.
Phoenix will need to have something else than just DDR5.
We already know 64MB 3DV cache at 7nm is only 41mm2(
Link).
RDNA3 will use IC chiplets, so adding a 64MB chiplet for Phoenix shouldn't be a problem. If the size is 49mm2 for example, then you get 1143 good chiplets and with $8000-10000 per wafer It would cost only ~$7-9, add packaging costs and I think It shouldn't be more than $20.
That's pretty cheap in my opinion.