So the prices of manufactured goods tend not to fall so much when the demand for them falls. All that happens is that fewer are produced. Bolivian tin mines are nothing like so flexible. The miners can only dig out of the ground what they find there - and usually they find tin. If it ever does.
Prices tend to jump around a lot in the commodity markets.
But in the long term many of the prices have been falling. Tin, for example, is being replaced for food storage by plastics. Copper, which as wiring is at the heart of most electrical systems, is being threatened by optical fibres. Cotton is being replaced by petrochemical-based fibres. Artificial sweeteners are increasingly being used instead of sugar in manufactured food products.