I’d go with something like this Lenovo M900 for $80-$90 used. Sometimes you’ll get lucky and find one with a single 8GB stick, then you can get another for ~$20 and you’ve got a quad core x84-64 machine with 16GB of RAM for right around $100.

The downside would be power consumption compared to a Pi if you’re trying to fit in a severely power-constrained setup, but the M900 will draw around 11-13W idle and 55W under higher load. so it’s not bad my any means.