It's not just rare earths... lots of minerals (most glamorously, gold) are this way. There are mines that are profitable to operate if the mineral is selling fo

It's not just rare earths... lots of minerals (most glamorously, gold) are this way. There are mines that are profitable to operate if the mineral is selling for $x per kilogram (or gram, or tonne), but are not profitable to operate if it's selling for $x-1 per unit (or any other figure less than x). Shutting mines down and (sometimes) starting them back up based on economic reasons has been going on for centuries.