Cheapest gigabyte/$ option for readily accessible storage like ssds/hhds?

I have been using an 8 tb external for my home lab for a year and a half now and am wanting to expand. I have been thinking of getting a usb c/3.1 hard drive rack and just buying a pallet of 20 4 tb used hard drives and stress testing all of them until i get a solid 10 or so that dont fair under stress testing. Does anyone have a solution? I was thinking e waste recycling plants or amazon liquidation warehouses but I don't know where to start. Thank you.