The trick is to get the lightly used 12TB drives from the people who just upgraded to 20TB drives.

But even if you buy new it's not really expensive per se, the issue is more when expenses happen all at once. If you expect that $350 18TB drive in the post to last about 5 years, that's $350/5/12 = $6/month, which really isn't that bad (rough estimate assuming the value will drop to 0, and ignoring opportunity cost; also energy is free)