Lithium is cheap because the externalities of the environmental damage it causes is not accounted for in the pricing. It's a highly exploitative resource which has destructive impacts on local bacterial ecosystems, human communities, and water availability.

Some articles, if you are interested:

https://www.sciencedirect.com/science/article/abs/pii/S09626... https://www.euractiv.com/section/energy-environment/news/fac...

It's not even comparable to sodium, which is abundant practically everywhere.