There are server chips like the E7-8891 v3 which lived in a weird middle ground of supporting both ddr3 and ddr4. On paper, it’s about on par with a ryzen 5 5500 and they’re about $20 on US eBay. I’ve been toying with the idea of buying an aftermarket/used server board to see if it holds up the way it appears to on paper. $20 for a CPU (could even slot 2), $80 for a board, $40 for 32gb of ddr3 in quad chanel. ~$160 for a set of core components doesn’t seem that bad in modern times.

I think finding a cooler and a case would be the hardest part