You shouldn&#x27;t expect the usefulness of devices to be linear in cost, even for completely rational buyers uninfluenced by fashion or culture.  Such buyers will select a phone where the <i>marginal</i> cost is equal to the <i>marginal</i> benefit.<p>Water gives my 99.9% of the usefulness of Coke (i.e., I don&#x27;t die of dehydration), but it&#x27;s also very reasonable for me to spend 10 times more for Cokes because the marginal cost is still small compared to the enjoyment benefit.