This illustrates a general principle that applies to all consumer demand. In fact, it is so important and general that we might call it the fundamental principle of consumers' demand. Here it is:
We remember the Law of Demand: a higher price means a lower quantity demanded, ceteris paribus. We also remember the Law of Diminishing Marginal Utility: each additional unit of consumption adds less to utility than the previous one. Since benefits are approximately utility in money terms, that also applies to benefits -- each additional unit of consumption adds less to total benefits than the previous one. So we have diminishing marginal benefits, and we can now see that the Laws of Demand, Diminishing Marginal Utility, and Diminishing Marginal Benefits all really are the same law, looked at from different points of view.
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