The cost of production is simply the cost of the (produced) factors of production (see Böhm-Bawerk's "Value and Price", part of his "Capital and Interest" works

The cost of production is simply the cost of the (produced) factors of production (see Böhm-Bawerk's "Value and Price", part of his "Capital and Interest" works). So the answer to your question is that miners ought to leave when the market value of their mined product is less than the marginal cost of production (assuming they keep track). If you have already recovered the cost of the mining tool, then the marginal cost is determined by power (electricity, cooling) and difficulty. At that point you'd be better off turning off the mining tools and using your money to buy BTC rather than power. On the one hand, BTC can go a long way down before that point, and difficulty would change; on the other hand, if the market price were to fall so much as to stop mining it would be tantamount to a complete destruction of the BTC system; I'm not sure how that would happen, it would surely not be simply a question of BTC pricing.