Compared to things like lithium-ion batteries or even the lead used in lead-acid batteries, I bet it's dirt cheap.

I'd say it's the manufacturing costs that will be the unanswered question.

Can't you just pour liquid concrete into moulds, and wait a few weeks? Maybe I've just been in South East Asia too long, but there seem to be few problems where pouring large quantities of concrete isn't considered the go-to solution.

Depends the point of view. Cement production consumes a lot of energy...

Depends on how you value the environment or human rights... https://www.wired.com/2015/03/illegal-sand-mining/ https://en.wikipedia.org/wiki/Environmental_impact_of_concre...