> I don't think LPDDR5 is that much more expensive than DDR5 on a per-GB basis.
It's also worth pointing out that while Apple is using LPDDR5 and getting more bandwidth out of it by just using more pins, that isn't that most GPUs use -- they use some version of GDDR, which is more expensive on a per-GB basis than DDR, because it has more bandwidth per pin.
So putting some "fast" memory on the APU package has the further advantage that you don't have to do what Apple did, you could actually put HBM or GDDR on the CPU package, and then the faster memory would be faster.
It's also worth pointing out that while Apple is using LPDDR5 and getting more bandwidth out of it by just using more pins, that isn't that most GPUs use -- they use some version of GDDR, which is more expensive on a per-GB basis than DDR, because it has more bandwidth per pin.
So putting some "fast" memory on the APU package has the further advantage that you don't have to do what Apple did, you could actually put HBM or GDDR on the CPU package, and then the faster memory would be faster.