You can also put an LSI SAS raid controller card flashed into IT into a PCI slot and use SAS to SATA cables. Can easily find them used on ebay for reasonable prices. And if you really grow your server, you can transition those SAS ports to point to a JBOD array with SAS ports, although that takes you from "cheap" to "cheap when compared to buying new."
I'm also a fan of Unraid since it makes expanding the array much easier, but you have to pay for it, and it's designed with the assumption that the only thing you're doing on the bare metal is storage, and everything else is either containerized or in a VM.