Is there still a whacking great price premium between ECC and non-ECC memory for desktop, but not server, motherboards? If so then that suggests that there's more going on here than a simple expression of consumer choice.
Yes for both.
Price: On pcpartpicker a stick of 16GB DDR4-3200 memory without ECC costs $28.99[2], but the ECC version costs $52.99[1]. Even factoring in that ECC ram comes with an extra chip, it's still 62% more.
Performance: using the same links above, we see that ECC ram tops out at 3200 MT/s, whereas for non-ECC you can get 4000+ MT/s. The brand selection is also better, which is critical if you want to get high performance memory chips like samsung b-die or micron E die.
[1] https://pcpartpicker.com/products/memory/#b=ddr4&E=1&sort=pr...
[2] https://pcpartpicker.com/products/memory/#b=ddr4&E=0&sort=pr...
Notable is that the max speed of 3200 MT/s for ECC is due to that being the fastest JEDEC standard for DDR4, you're not getting the XMP-profiles to run them faster and will have to resort to manual overclocking.