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.
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.