I got an ecc-ready used workstation on Ebay. After adding a smallish SSD for the system drive and upgrading the memory to 16GB I think my total cost was in the neighborhood of $200. Cost about as much as two top-end fully-equipped (case, heat sinks, disk) Pi4s, but is much more powerful, and, conveniently, includes space & ports for my SATA spinning-rust bulk storage disks.

It is a (fairly small) desktop tower, so it takes up quite a bit more space than a couple of Pis, though, again, it also encloses some internal hard drives, which is nice. I'm not sure about power use but I'd just gut-instinct guess it's equivalent to four or five Raspberries Pi, even if you take out the power to run the hard drives, so it is (probably) worse on that front.