The absolute cheapest way to get storage. For the barest bones, ugliest setup, I've seen people have two PC power supplies on their desk, a motherboard, a few SATA cards, and a number of drives plugged into the SATA cards, and using Linux + ZFS + Samba for the heavy lifting. Alternatively, a "NAS PC case" with a decent motherboard and such should work.
If I were building the cheapest way to have a lot of storage, but have a warranty, I'd go for a higher end QNAP NAS that supports QuTS Hero, even QES. I would then load TrueNAS SCALE on the QNAP hardware, use ZFS from there on out. This ensures a lower attack surface, and ZFS without any added stuff. The QNAP hardware isn't cheap, but it is fairly reliable.