According to this PDF: https://www.eia.gov/electricity/sales_revenue_price/pdf/table5_a.pdf

The average monthly electricity bill in the US comes to between 90 and 130 USD

5 * 30 (average days in a month) is 150.

So I'm not sure how you're calculating $100 profit. It's more along the lines of 30-50 profit.

With even a monthly profit of a mere $50, the cost of the cards during the mining craze being in the 400-500 range, you'd need to be mining for months to remotely make a profit.

I wasn't targeting this specific person, if they are using their own cards, but people are literally spending 100's of dollars on GPU's to mine.

Profit wouldn't even happen until at least 6 months of 24/7 mining.

And even that is subject to the fluctuation of the bitcoin market. This thread is talking about how the value has dropped.

It's a pittance, for the sake of running your GPU to the max, 24/7.

You'd also need to contemplate replacing the fans after you're done with the mining, if you want the card to last. That's another cost.