> The manhole itself has no inherent extra value over its material outside of its context.

Yes.

> Melting it down is money for the thief, then for the drug dealer, but also money for the scrapper, the ironworks, the miners providing coal to the ironworks.

Ok, so let's build a giant manhole cover factory for the purpose of dumping them in town so they can be stolen and melted back down and the cycle can power the economy.