Bitcoins mining difficulty adjusts every couple weeks (every 2016 blocks) based on mining power increases / decreases. so the production cost is dynamic.
so we can’t use production cost as a bottom because there’s never a static cost. if the price dropped below the current cost, mining wouldn’t profitable so miners drop out.
when they drop out difficulty decreases (lower hash rate). production cost lowers because less electricity needs to be used to generate btc. miners join back in because it’s profitable again in terms of electricity cost to the new price, but price drops another $1000 from speculators (majority of btc owners) selling.
miners (who aren’t speculating & willing to take a loss) drop out again. production cost (bottom in this scenario) drops again. repeat all the way down.
It’s all about speculation / future expectation.