I disagree with you on HP Mats and still think my explanation is better: The rate never mattered. Every character regardless of build required 125 HP Mats, hence the higher demand and higher price. They've dropped in price because many players are maxed now. Not because their drop rate was discovered. An item's value is a combination of its rarity plus value. If say, 2.5x HP Mats are needed over Pow Mats, which is a pretty reasonable guess, then the previous 2:1 vs. 5:1 made sense.
What further supports this is that HP mats (and TP Mats) are the only mat types that aren't destroyed when a material reset is performed. Thousands of the other mat types have been destroyed. So, the demand for those stat materials would have been at least somewhat affected by mat resets, whereas the value of HP Mats would not have been.
Regarding your remarks on PCs: I asserted that the drop in PC value was not due to PD inflation. What you said agrees with this. So, yeah...
Edit: I find economic analysis, even in a fictitious setting, to be interesting. So sorry to anyone who thinks this is dumb .-.