This 7th gen Celeron is $45 https://www.amazon.com/Intel-BX80677G3930-Celeron-Desktop-Processors/dp/B01MYTY55V/ref=sr_1_1?ie=UTF8&qid=1498060262&sr=8-1&keywords=Intel+Celeron
but depending how small factor you want it a Atom X5 would handle your emulation needs no issues but it won't be it won't be upgradeable you'll be stuck with that chip even if you aren't satisfied with it. I've bought a few cheap PC only to get new ones because I'm unhappy with the performance.