Miners adapt their mining expenses according to bitcoin price, not the opposite. In the hypothetical case that bitcoin goes to $10, miners would spend less than $10 to mine each bitcoin.
I thought the expense was to due to rising difficulty, and that determines the cost of production, which in turn help determine the market valuation..
Mining difficulty adapts to bitcoin price. If price drops to $10, difficulty will drop too.
Is just simple offer and demand. If market don't want to pay more than $10, miners can't spend more than $10 to mine them. Otherwise they would be mining at a big loss.
But if market want to pay $10,000 and miners only spend $100, that's ok because they are doing it at a profit. Of course more miners would join mining because high profits and then, after time, mining each bitcoin would be near $10,000 each.
Hint: Remember bitcoin could be mined for nearly $0 each (if just 1 computer mining and no demand at all). It is just offer and demand (rising price and difficulty) the one who makes mining more expensive.