> Rack mount hardware is almost always expensive, loud, and power hungry.
It really depends on the particular hardware. I recently picked up an R720 with 128GB of memory and dual E5-2670v1 for $450 on eBay. It idles at 120W (about the same as my brand new Ryzen 5950X desktop). It is not much louder than my old air-cooled 8700K consumer PC. Of course, it's not much faster either, and it's definitely slower than my Ryzen.
I bought it to learn how to use iDRAC and practice ZFS with 10x $20 1GB 10k RPM SAS drives. Also maybe to give Proxmox a try. All of my practical home-prod stuff runs on an old i5 desktop, not my rack servers.
The issue is that power savings stuff is often bad on Linux... especially on older hardware. 'Idle' in this context means running with near zero load since a server cannot suspend or hibernate. In theory you could setup hibernate/suspend + WoL but that tends to not work well.
Some power saving features will really only save 5-10W and can make PCIe cards unstable, poor hard drive performance, laggy keyboard & mouse, etc.
In particuler, GPUs often have poor suport for the normal power saving features. I just pulled the dedicated GPU (GTX 760) out of my old machine that is now a headless server and that saved ~20W. What's weird and bad is that that 20W was being consumed without a graphical interface even running.
It really depends on the particular hardware. I recently picked up an R720 with 128GB of memory and dual E5-2670v1 for $450 on eBay. It idles at 120W (about the same as my brand new Ryzen 5950X desktop). It is not much louder than my old air-cooled 8700K consumer PC. Of course, it's not much faster either, and it's definitely slower than my Ryzen.
I bought it to learn how to use iDRAC and practice ZFS with 10x $20 1GB 10k RPM SAS drives. Also maybe to give Proxmox a try. All of my practical home-prod stuff runs on an old i5 desktop, not my rack servers.