Every time iron ore prices move by one dollar a tonne, it means another $120m on – or off – this year’s post-tax net profit at BHP Billiton, the world’s third-largest producer of the metal. Last year at Rio Tinto, the second largest, $120m of earnings also rode on, or off, each $1 move.

Mostly off, then, when the spot benchmark iron ore price falls $10 over a weekend. Yesterday’s drop to $104 a tonne may reflect bad or worse signals from China: weak economic data, or genuine signs of financial distress in a bloated steel industry. Whatever the cause watch the bottom lines of Rio – dependent on iron for four-fifths of its earnings – and BHP. The plunge (in both the iron ore price and the miners’ share prices) has also spoiled the end of an earnings season in which miners touted their success with cost cuts first announced when iron ore prices last fell hard in 2012.

Here we go again? Yes, miners can go on promising to reduce costs and slow down capital spending. But this is a difficult, time-consuming process. In the minds of investors, commodity prices can drop harder and faster. That’s a problem for a smaller player like Fortescue, which is more exposed to short-term pricing than Rio or BHP.