That is not true. The last time I have looked ECC DDR5 UDIMM modules had a price higher by at most 50%.
Nevertheless that is still excessively high. While in the beginning for DDR5 there were only 80-bit modules, which could claim a +25% higher price, now there are 72-bit modules, like in the previous generations, which can justify at most a +12.5% price increase.
I looked right when I posted that and found 2x32GB non-ECC 5600MHz for $165, exactly half the $330 price of the sticks listed in the post. I spent several minutes looking for cheaper ECC at the same specs and couldn't find any.
Trying again, I can find some Kingston sticks that are $120 each, so that's about 50% higher. Amazon's search is really bad, by the way. But that's not the "at most" price. And a month ago they were $140 each.
Edit: Micron data sheets suggest that UDIMMs have 2x13 command/address pins and RDIMMs have 2x6, so that's one piece of the puzzle. Apparently UDIMMs can do x64 and x72, and RDIMMs can do x72 and x80.
When DDR5 was first introduced, there were only x8 chips.
Because the DDR5 channels must have a width of 32, 36 or 40 bits, with x8 chips one had to use 40-bit channels, even if only 36-bit channels are needed, so indeed 10% of the memory capacity remained unused.
Meanwhile, about a year ago, at least Micron has also introduced x4 chips. There are such ECC UDIMMs, using both x8 and x4 chips, which waste no memory.
On the market there are both modules made only with x8 chips, which do not use a part of the memory, and modules with a combination of x8 and x4 chips, without unused memory.
