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There is 280x difference in price between iron and tungsten.

https://www.metalary.com/tungsten-price/

2000 tonnes of tungsten is 60 million USD, of iron is 200k USD.

Tungsten costs around $30k/ton, so you'd be looking at a cool $15M for that 500T cube.

Lead goes for around $2k/ton, so this might be a more feasible compromise.

Depleted uranium prices are very hard to find, but should come as cheaper than tungsten.

Civilian sales look at least possible, I heard of at least one sailboat with a DU keel.

This is a technology where energy density doesn't matter at all because the amount of energy stored per ton is the same. So you want as many tons as cheap as possible. Therefore we either use water or concrete.