I don't know where people get this idea that HBM is some super-expensive memory: it isn't.
If
this is correct, then even in 2020, a GB of lower-speed HBM went for around $7.50. Now, the upper limit of HBM 1 is 1 GT/s, or 128 GB/s for a 4-Hi package (4-Hi being the limit). I'm sure HBM2 would cost more per GB, but not astronomically more, especially considering Sony would only need to hit PS4 bandwidth of 176 GB/s.
A 4-Hi stack of 8 GB low-speed HBM2 would probably not cost them more than $45, at the volumes Sony could order. The interposer may add a bit more to that, but again, at good volumes, not anything dramatically north of an additional $5. We're talking about a portable that'd probably have a MSRP of $399; that leaves more than enough room for this memory, as well as all other necessary components, while still selling it for a pretty decent profit margin out of the gate.