Well, if you want a 2TB SATA SSD, count on paying ~$550 for an MX300. If you want a 2TB 960 Pro, I think you'll shell out $1,200. But you can get a 3TB HDD for less than $100.
Suppose you already have a 512GB or 1TB 960 Pro or EVO. If you can live with 400GB of a system volume for the smaller unit, you have something around 100GB left for caching. $30 for software (and there's more than one alternative to the one I favor), and $100 for the HDD. $130 to make that storage seem a lot faster. But if it seems faster -- what does that make it, really?
That's going to be a difference you can "detect." Suppose, instead of the HDD you get a 1TB SSD unit for $250 -- Not a Sammy, maybe a Crucial or ADATA. And, you cache it, too, to the NVMe 100GB volume. Howsoever you cache it to NVMe, you won't notice any difference between the HDD and the SATA SSD except for the write speeds. If you dare yourself, you can fix that too.
It's a long-established strategy going way-back-when, but your IT veterans think it makes more sense for server systems. While I don't like weighting my midtower with a handful of 3.5" drives, I not only want fast essential (system-OS) storage, but I want cheap, high-volume storage. I'm using 3x 2TB 2.5" "laptop" spinners in my desktop with the 5,400rpm spec, in addition to my NVMe drive. Do I care if they have the spec lower than 7,200? No. And anything between 120MB/s and 300MB/s of electromechanical storage can be lifted in performance high enough that it doesn't make a difference whether they're the fastest VelociRaptors around. As for those lappie drives, I cache one of them to the NVMe, a second one to non-persistent RAM-cache, and the third is just a backup drive for daily Macrium images.
But in all of this, you also have folks choosing between 2x4 8GB, 2x8 16GB and 2x16 32GB RAM kits. I find that 16GB gives me a comfort-margin, but it's not frequently utilized -- it's just "available." A lot of folks get by with 8GB and there are plenty of lappie and even desktop systems out there with 4GB. I can spare AT LEAST 4GB for RAM-caching. What could I do with a 32GB kit? It could almost make the Optane seem inconsequential, but it's not PERSISTENT cache, except for the software's feature to save the cache on restart. There -- you have to wait a little bit at boot time, except for the speed of your boot disk. Then, you would fret over the number of writes to your NVMe boot drive.
It has become such an interesting proposition and question for me that I've had my eye on a 32 GB pair of 3200 14-14-14 G.SKILLs and the 2TB MX300 for about 4 months now. I can't decide what to do with $1000 sitting in my budget and account to spend. And the performance of the 960 NVMe together with the 2TB Barracuda and a 16GB kit of RAM totally dispels any frustration I have about it. Those products will be available even if I wait through the whole year.