The choice of numeraire is shown to be important in cost-benefit analysis. When a public good is involved, individual consumers' marginal rates of substitution will generally differ. Thus, the less valuable the numeraire is to a person, the higher the number required to express his net benefit, and the more will his interest weigh in the total sum. The choice of money as numeraire is systematically favourable to those who value money the least, relative to alternative numeraires.