Not sure if this helps, but I very recently reached out to Small Green Computer, and they had some refurbished microRendus for $350 USD (plus the cost of whatever power supply you decide to go with). So not as cheap as a Pi 4, but not terrible, and I’ve seen them even slightly cheaper than that on the used market.
It’s a very simple device though: no screen, no wireless, only USB output, etc., so it also depends on what you need.