I was pricing out replacing my NAS disks recently and found that 14TB disks have the best capacity/price. You're right in that 20TB and 22TB drives make no financial sense being twice the price of an 18TB drive, but if you're looking for the lowest priced storage for an array, then you need to look at disks smaller than 18TB.
You can get refurb 18TB for $250 (13.8 USD / TB) vs OEM 14TB for $209 (14.9 USD / TB). The refurb comes with a 5 year warranty, the OEM does not. You can find 14TB refurbs for cheaper but they are older and often only come with a 1 year warranty due to their age.
Most of the enterprise world is on the higher capacity drives which means you can find plenty of refurbs for cheap and they are new. These are often drives that just had a failed motor and the refurb testing process is more strenuous than what a new drive receives.
Considering that the 18TB drives have a higher density, speed, and price when you know where to look I'd definitely call them superior. You are much less likely to get old stock with an 18TB drive to boot.
Also do note, many drives sold on Newegg and Amazon are OEM drives even if the product page says it comes with a warranty. Always check your drive serial number and always run a new drive through a full write / wipe to ensure proper functioning of the mechanics and ensure the disk surface doesn't have any imperfections. Preferably with real data although there are synthetic tests around for this purpose.