Wednesday 31st May 2017: 8.08pm.
As I do this time every year, here is my time series graph of spinning rust storage per inflation adjusted dollar as against flash storage, updated with data for 2017. As you will note, flash storage actually is more expensive this year than last by 5-10% probably due to the global flash memory shortage, and I also see that Intel's NVMe SSDs are as competitive as their non-NVMe SSDs for the first time, so I used the NVMe products. NVMe SSDs run rings around their SATA brethern in throughput and latency, but chances are you won't notice it most of the time, this MacBook Pro I'm typing on can sustain 3Gb/sec to its NVMe SSD, yet I can honestly say it doesn't feel particularly faster than any other SSD based laptop I've used.
Hard drives have been stuck around 1.25 Tb/platter for a good three years now, so I would expect the next leap upwards will land on the market soon. The trendlines indicate that flash will still never catch up with spinning rust in capacity per dollar, and unless Intel's new flash replacement technology X-Point can ramp up much steeper than flash can, spinning rust drives are here to stay.
Raw data:
http://www.nedprod.com/studystuff/SSDsVsHardDrives.xlsx