Been a Monero miner for a few years now and wanted to scale up a bit for fun. Purchased some used 3900's from ebay, new gold psus, b450 mobos, 2x8 3200 cl16 memory kits, and usbs to boot from. I think I spent about $400 usd per system for 4 systems, total $1600. I could've bought used for the rest of the parts (especially for bdie's instead of cdie) and saved quite a bit, but frankly I was too lazy. I've been working on building the rest of the systems, but the one I have running right now is running 3.5ghz @ 0.95v on the cpu, and 3333 cl 15 @ 1.4v on the memory for 12.5kh/s. I spent a while overclocking subtimings but in the end concluded the time investment wasnt worth the minor gains and decreased uniformity. At current rates, I would ROI in about 5 years. Not too shabby considering I own the systems and can sell them for most of the money back if I ever need.
Currently running Alpine Linux on each of the miners, and I have a separate 5600x server running NixOS that I use to run my nodes (among other applications). I was going to use Nix on the miners as well, but Alpine is easier considering its a single user machine, and probably faster. Of course I'm running my own monerod and p2pool, might throw xmrig proxy in there for stats as well but that can be a future endeavor.