Real question right here

Probably quite a bit. November 2021 is around the peak for Bitcoin prices and it was relatively high for others too. Assuming it was running for a few months before, you'd probably have made a decent chunk with a dozen or so computers mining 24/7 for "free".

$18k of electricity is also just the running costs. The photo in the article shows at least a dozen computers of sorts, each of which likely required a few hundred to a few grand in computer parts to successfully mine crypto. I'd assume they still profited after purchasing/acquiring all of the computers though, since the electricity costs are usually the killer for crypto mining.

IDK what computers/devices they were using though. NiceHash says some of the top ASIC miners today could be making $29 USD per day. The top one is like a $7k device though. But assuming this guy dropped his life savings in this scam, maybe he managed to generate a few grand a month. Since $29 * 12 * 30 is like $10k, so maybe a few months of secret mining got him his investment back and he pocketed a chunk of change. And that's with the current crypto pricing slump - Bitcoin has dropped by like 3x since November 2021. So maybe this guy made upwards of $30k/month if he had the right mining hardware and the price of crypto in his favor.