You make HBM by stacking a whole bunch of dies on top of each other. The signals from the upper dies need to pass through vias in the lower dies to get out - ta

You make HBM by stacking a whole bunch of dies on top of each other. The signals from the upper dies need to pass through vias in the lower dies to get out - taking up valuable die space in a way which simply isn't needed with regular DDR. Similarly, HBM has a far wider bus, so each individual die has, say, 16 banks of depth 32, rather than 4 banks of depth 128. That's more control area needed per byte of memory. Those two combined already result in a huge reduction in bytes per mm2, so with the same wafer processing capacity you're producing far less byte of memory. Add to that a complicated chain of HBM-specific packaging steps, and you're now also losing a decent bunch of perfectly-fine dies because rather than putting it into DDR you tried making a HBM sandwich and screwed up. Even if the memory cells are the same and have an absolutely identical yield, HBM will always end up having a significantly lower output. That's just the cost of stacking, but some people are willing to pay the per-gigabyte price penalty in return for the higher bandwidth.