www.phison.com
This is the controller going into Series X, it's 4 channel, so I assume, they are either using four 256GiB or two 512GiB NAND flash modules to get to the 1TB storage.
Sony on the other hand are building their custom 12 channel SSD controller, from what I've read on this topic, it's expensive, but then I also remember Sony removing many of the features from a regular SSD controller which are not needed, thanks to tremendous amount of resources they must've poured in R&D. Due to higher speeds, I think they are using more NAND modules in parallel. I guess, twelve 64GiB modules to get to the 768GiB aka 825GB storage.
Now, if any of you have access to this site,
https://www.dramexchange.com/ we can hazard a guess at total SSD cost. But, I imagine, it should be slightly on the higher side of PS5.