Golf cart versus BMW
This type of comparison is too shallow. SSDs are so much faster that you can compress data in real time, reducing the cost/TB by around 5X. I suppose a tiered SSD/HDD hybrid might get that advantage, but then we need to factor the reduced number of appliance boxes we need with 2.5" SSDs or the new ruler drives, versus what will always be 3.5" HDDs.. I figure this gives SSDs a reduction $.10/GB.
Next, the massive performance difference reduces server count dramatically, perhaps by 3x on average. That yields another $.20/GB and operating cost savings will cut $.10/GB or more.
As 3D NAND ramps in 2018, with QLC coming to the bulk secondary storage market we are comparing, the numbers are a lot closer, especially with the new fabs Samsung and others are opening. HDD capacity is stuck till HAMR comes at end 2019 and HAMR will increase $/GB.
Bottom line is the compairon is loaded to support HDDs.