The 18 TB Western Digital Elements USB drives dropped in price, today.
Amazon has them for $339.99 state side seller (limit: 3).
*** Edit: I thought that this (below) was a UK seller. I might be mistaken.
Amazon has them for $339.99 UK seller (limit: 3).
B&H has them for $339.99 (limit: 2).
https://www.bhphotovideo.com/c/product/1604997-REG/wd_wdbwlg0180hbk_nesn_wd_elements_desktop_18tb.html
That’s $18.89 per TB, which is the lowest I have seen, over several months, for external drives of this capacity.
Cheers!