At $44 for 16GB or $77 for a 32GB module Intel's Optane memory will cost you less in total for an M.2 SSD, though a significantly higher price per gigabyte.  The catch is that you need to have a Kaby Lake Core system to be able to utilize Optane, which means you are unlikely to be using a HDD.  Al's test show that Optane will also benefit a system using an SSD, reducing latency noticeably although not as significantly as with a HDD.

The Tech Report tested it differently, by sourcing a brand new desktop system with Kaby Lake Core APU that did not ship with an SSD.  Once installed, the Optane drive enabled the system to outpace an affordable 480GB SSD in some scenarios; very impressive for a HDD.  They also did peek at the difference Optane makes when paired with aforementioned affordable SSD in their full review.

"Intel's Optane Memory tech purports to offer most of the responsiveness of an SSD to systems whose primary storage device is a good old hard drive. We put a 32GB stick of Optane Memory to the test to see whether it lives up to Intel's claims."

Here are some more Storage reviews from around the web:

Storage