You could use a website like WhatToMine[0], together with an estimate of the hashrate of the specific CPU you are looking at, to figure out how much money you could generate with said CPU over a day of mining at 100%. Estimates can be obtained from crowd-sourced websites like monerobenchmarks[1].

Focus on CryptoNote-based coins: Monero, Electroneum, Sumokoin are some of them. These are still profitable with modern CPUs. Hashrate of a given CPU will be the same over any CryptoNote-based cryptocurrency.

[0] https://whattomine.com/ [1] http://monerobenchmarks.info/

Edit: Just to add some real numbers. My home desktop is based on a Ryzen 1700. It is currently overclocked @ 3.4 GHz. I can get mine around $1.60/day worth of Electroneum at current prices. Estimated TDP is <80W (CPU alone), which at SoCal electricity costs results in $0.30/day. Profit is then around $1.30 a day.