128GB of ECC RAM itself is going to be ~800-1000$ as far as I can find some to fit in a Ryzen build. So it won't be trivial to reach the price point for a full build although Ryzen is clearly great value anyway.
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.