That $1500 includes the rest of the computer, though. That adds up.
Here is a 128gb kit for under $600 dollars. https://www.newegg.com/corsair-128gb-288-pin-ddr4-sdram/p/N8...
If your workloads require 128GB of RAM then you probably want ECC. You'd be skimping in the wrong place.
big "if" there really, and that makes it an apples-and-oranges comparison IMO.
The motherboard, RAM, SSDs, CPU, case, powersupply all have to be factored.
This is an apples to oranges comparison.
IMO it always makes more sense to build your own PC vs buy a used server.
It's going to be have better ventilation and be much quieter in a home setup.