The long term relative prices of M.2 NVMe drives and 2.5" SSDs
For reasons outside the scope of this entry, I recently found myself wondering if in the end, M.2 NVMe drives will wind up being less expensive at moderate capacities than 2.5" SSDs of the same capacity (regardless of the 2.5" interface interface involved, which might be SATA, SAS, or U.2 NVMe). I reflexively think of M.2 NVMe drives as a better, high end product that is and always will be more expensive than ordinary 2.5" SATA SSDs, but the more I thought about it, the more I suspect that the economics tilt the other way in the long run.
The reason why is what we could call the tyranny of physical stuff. Both M.2 and 2.5" SSDs have the same basic electronics; they need flash chips, some DRAM (hopefully), a controller, a PCB, and assorted bits and pieces on the PCB. But 2.5" SSDs also need a case and some extra connectors. Those extra components have a cost, and eventually that cost (and the cost of assembling them) will probably dominate over other things like the relative cost of controller chipsets.
(I suspect that SATA SSD controller chipsets currently are cheaper in bulk than NVMe controller chipsets, partly because NVMe controller chipsets keep iterating for things like PCIe 4.0, while SATA SSD controllers have a relatively static job.)
The 2.5" form factor provides more room for internal electronics, but anecdotes are the modern 2.5" SSDs are mostly empty air inside the case. Right now, I believe they have an advantage over M.2 NVMe for higher sizes because it's hard to squeeze that much flash into the modest space of the M.2 form factor (and you may have to use more expensive, higher density flash), but flash chips seem to be relentlessly marching to higher and higher densities in general. Plus, a lot of drives are relatively modest sizes, especially if you're looking at the server space. The 2.5" form factor may always have a price advantage at really large sizes, just like HDs so far have that advantage over all SSDs, but that seems likely to be less and less of the market over time.
In fact, now that I look at pricing online, it seems that this may have already happened without me noticing. At the very least, it looks like M.2 and 2.5 SATA SSD prices are roughly the same for moderate sizes, and there may be more M.2 NVMe drives (or sometimes M.2 SATA) than 2.5" SSDs listed. On the other hand, any number of the M.2 drives are from brand names that I barely recognize, while most of the 2.5" SSDs are from relatively well known names. It may be that the M.2 market is where everyone thinks the opportunity is for really low end, compromised products.
(I checked one well regarded brand that sells both M.2 and 2.5" drives, and for their smallest drives the sale price on the retailer I looked at was the same, although the list price of the M.2 NVMe version was slightly higher.)
PS: It's likely helped the M.2 form factor that it's what laptops have used for a while if they're going to have separate drives, and laptops still have a decently large volume. Although I don't know if more M.2 drives are made and shipped than 2.5" SSDs, even with laptops. But I may be biased because we get servers and use 2.5" SSDs in them (so far).