The best bang for buck at the moment seems to be tiny mini micro machines https://www.servethehome.com/introducing-project-tinyminimic...

Typically available regularly via ebay (or similar) as businesses rotate them out for new hardware.

The other week I picked up an i5 9400T Lenovo m720q with 16GB of memory for £100 delivered.

They practically sip power, although that's less true now I've shoved a 10Gb dual SFP NIC in there.