Where are you buying 20TB HDDs for $150?
There are still deals to be found that are closish.
Microcenter has a 24tb external for $249 right now: https://www.microcenter.com/product/679395/seagate-24tb-expa...
amazon has a 22tb for $229 right now: https://www.amazon.com/Seagate-Expansion-22TB-External-Drive...
Those and my $189 price - which is definitely higher than I originally claimed, apologies - are all around $10/tb.