I got a cheap atom ("Celeron" quad core) NUC with 16 gb SODIMMs (already had) and a 512gb SSD... It's more than enough for an office computer.
https://www.newegg.com/intel-boxnuc6...9SIAD6HB4A4712
$200, add your own ram/hdd, and it has a 15w TDP. Iirc it idles around 7-8w, running all out it hits 25w or so.
Alternately, for my media/gaming machine in the living room, I used an AMD 2400g (now there's now a 3400g update). 65 watt TDP including pretty solid 3d performance and quad core/8 thread. AMD also makes a 3400ge they sell to oems only that has the same specs but a 35w TDP.
But for a cheap, energy saving office machine, you can't really beat the NUC.