The RAM in the M5 is LPDDR5X, not HBM. You're right of course that it's unified and used as VRAM in that sense.

HBM itself is very expensive but it’s not really fair to compare to LPDDR or GDDR

They’re very different things.

The more logical argument to me is that Apple uses its upgrade price points as more than just direct BOM and rather as a proxy for things that are amortized across all their sales like support/warranty/etc so higher SKUs subsidize the costs of the lower ones.