This wasn't my experience either on memory.net or crucial.com
> expensive
Yes it is, much more than the 1/8th more RAM would require.
> much slower
to quantify, the fastest currently is DDR4-2666 while non-ECC goes to the crazy DDR4-4700.
As far as I can tell crucial.com shows a total of 2 options. Both 16GB UDIMMs. One tall, one short. I think last time I checked it had none. I didn't see a single option on memory.net. Remember that you need Unbuffered/Unregistered ECC (UDIMMs with ECC) and there aren't many options of those. RDIMMs will not work.
It's also hard to verify the DIMMs will work because the motherboard's QVL doesn't list any of the ones I've found so far. Not all retailers will have these either, so that seems hard enough to find for me.