That is rather stiff pricing but it is not like the 4 regular SSD slots weren’t a dead end - they had to do something about that.
If I would be getting one of those I would probably use it for special stuff and let the regular HDD Raid take care of the rest as U.2 SSD prices seem to have gone up in the years since I last checked. My choice might be to get one of those U-2 to M.2 adapters and combine with a couple of WD SN850X M.2 that at least go up to 8TB and their per TB price is almost half of what they charge for the U.2 drives.