Secondhand HP T620 is pretty cheap and comes with sodimm and m2 sockets. T610 has weaker processor and sata instead of m2, but much cheaper than T620. I'm running homeassistant and pihole in a T610 with 8gb ram and 256 ssd and it's pretty great so far.
Online specs says T610 idles at ~12W and full load at ~20W [1], but I haven't personally measure it myself. I'll try measuring it myself when I got a chance later.
If it's really idle at 12W, it means the electricity cost is about $12/year. For comparison, I think pi4 averages at ~5W.
