Money can be traded for people's time, physical stuff, or rental of resources (including money).
Compound interest is discussed on the NetPresentValue page.
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The cost of money is time + energy (where e=mc^2).
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In a Finance course I took years ago an important term is CostOfCapital. It makes the recognition that an organisation has finite resources and cannot undertake all worthwhile projects with expected positive NetPresentValue.

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Bernard Lietaer wrote  "the future of money"
He explains all about how money works
and how it could do better for the benefit of all.

http://www.transaction.net/money/

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The pursuit of money can have an opportunity cost. A particularly vivid case of this is documented in ''1493: New Revalations of the World Columbus Made'': China, being short on precious metal and high on wealth, exported vast quantities of wealth in order to import silver to provide a currency. And this after they had already invented paper money; if only they'd avoided hyperinflation, they could have stuck to paper as a store of value, sparing themselves the need to import precious metals -- which are not actually useful for very much, except as a stable store of wealth.

Pursuing fiat currencies can have an opportunity cost too; think of the seignurage benefits that accrue to the US as the issuer of the world's reserve currency. (And think of the issues with competitor currencies -- the Euro's quirky governance and smallish economic base, China's corruption and patchy development -- that keep them from supplanting it.)

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See also:  CategoryEconomics