If you already paid for your hardware (or already committed to paying for it), then there's zero additional cost, unless (i) there are significant maintenance costs or, more likely (ii) you could recoup some of the cash by selling the equipment if you stop mining. I'm ignoring time value of money, and also the drop in the resale value of the equipment. (Deliberately avoiding the work 'depreciation' here as the accounting meaning the every day meaning confuse the issue.)