First of all, on all your HDD price graphs there is a clear downward trend.

Secondly the way you are making the comparison doesn't make sense. On a device SKU level, a particular model of HDD is of course going to remain stable for the life cycle of that model because it's a complete monolithic design, a HDD factory kitted out for a particular model does just that and nothing else.

A SSD drive on the other hand is just a repackaging of flash chips, if Crucial manages to buy cheaper chips that fit you bet they are going to use them. And the chip fab can make many different chips, not just one model so it's not comparable economics at all.

What you need to do is compare price/TB on the current market, not of 3 year old devices.