Western Digital easystore 16TB external USB drive – $279.99
https://www.bestbuy.com/site/wd-easystore-16tb-external-usb-3-0-hard-drive-black/6427998.p
That comes to $17.50 per TB, which looks like the current low price per TB available today, for a high capacity external USB drive.
Their 18TB drives are $339.99, which translates to $18.89 per TB.