[NortySpock]

https://www.parkytowers.me.uk/thin/hware/hardware.shtml

From the above site I landed on the Dell WYZE 7020 / Zx0Q... Some models of which have 4 cores (also accidentally bought a dual core version), supports 16GB DDR3, are passively cooled without a fan, and cost me about $50 on eBay (add $20 for a 16GB RAM kit to bring the total to $70)

It's about as fast as a Raspberry Pi 4, and has been great for letting me spin up a docker container to play around with a database or a message broker or what have you.

I limited my search to only fanless designs though; if you are fine with fans then I'm sure there are more performant options out there.