Apple

$50 to maaaaybe $200 if you're really lucky. More likely on the low end than the high end. Macs from that era and even a lot of newer ones pre-M1 have nosedived in price because Apple isn't supporting x86 going forward.

I bought a 2015 for $110 6 months ago, so probably less than $100. It’d be worth more to keep as a backup if it is in working order versus selling it in my opinion.

Go on Craigslist for your city or a nearby large city and see what is listed. Check the specs vs yours and you'll get an idea