Your estimate was way over however. Basically the very last time that I remember when CPU mining was profitable enough for 20 CPUs to gain $500 a month was probably during the XZC launch days. Most of you probably weren't around but there was a small privacy coin called Zcoin, not Zcash.

Its launch was big because I think there was a rumor that Roger Ver was invested in it. Anyways the coin originally was CPU only for a while. And the way it was coded was that you could have a slow Celeron CPU and a fast i7 CPU and they for some reason would hash at the same hashrate. It was pretty much 1 CPU 1 Vote as Satoshi intended.

Anyways people back then were renting out Amazon AWS and were making crazy profits for a few weeks with that coin. It was actually the only time ever that I CPU mined with my Sempron/Celeron CPUs while GPU mining ETH. I think I remember I was triple mining in those days. Basically dual-mining Ethereum plus Decred and also CPU mining ZCoin. Those sure were the happy days of mining. Miss those days.