This is a complex issue, that is highly dependent on exactly what you want to do with that RAM.
In most cases, it is cheaper and better to simply replace the motherboard with a new motherboard that supports the amount of RAM that you require. I have a motherboard here in front of me that can take 16 memory modules. The largest module available is 32-Gig. That's a total of 512 Gigabytes in a single machine. (Never mind that 16 modules of that size would cost about US$14,000, or that the MoBo also has dual 8-core CPU's on it.)
Having the RAM on the MoBo means that it is the highest speed possible. You can use it for both a RAM-Disk as well as normal program and data storage. The best of both worlds.
But in your question you keep comparing it to SATA storage, so I am thinking that you'd want to use this extra RAM as a RAM-disk and not for general CPU RAM. This is a valid use, and years ago people did have PCI cards with lots of RAM on it specifically for this purpose. Those cards looked like another disk drive, and not just more CPU RAM. Often these cards had an external power connector on them so you could give them some sort of backup power in case the main power failed.
These types of cards have largely gone away. They were obsoleted mainly by three things: 1. Motherboards now can have much more RAM on them than in the past. 2. There are more modern solid-state drives using Flash memory and PCIe (some with large RAM caches) that work better. and 3. They were just too expensive for what limited advantages it gave.
There are other reasons why you might want to have a PCIe card with lots of RAM, but all of them are applications where the card is doing something other than just storing data. Like Video cards, or data acquisition cards. These things do not apply here.