One guy here said his profits already paid for his equipment and he does not pay for electricity because he gets it from a green energy source. He can mine for nothing until his energy generating equipment or mining hardware breaks. No doubt he's in a minority but there must be others like him.
At some point if the BTC/USD price drops far enough, it makes more economic sense to sell your solar power back to the power company rather than using it for mining. Unless you live off the grid or your power company won't buy your KWh.