Economics and its sub-disciplines often developed in close relationship with its audiences. This paper examines the public life of one particular case, the idea of marginal social cost pricing for ports, by highlighting two key episodes. The first episode was the emergence of port pricing in the 1970s, when the academic concept found a willing ear at international organisations such as the World Bank. The idea of marginal cost pricing for ports replaced the then prevailing principle in port policy of national public interest, with the principle of economic efficiency. The second episode was the eager adoption of the idea of marginal social cost pricing for ports by the European Commission in the 1990s. Along the way, from the 1970s to the late 1990s, the relation between economics and its audience appears to have undergone a dramatic shift. In the earlier episode, academic economists took the lead finding only a receptive audience with international organisations. In the 1990s, by contrast, one such international organization, the European Union, took the lead in advancing the idea of marginal social cost pricing, pushing economists into the role of either intellectual defender or critical observer. The insights from this particular history may be useful to obtain a better understanding of the role of applied economics in the micropolitics of the various waves of deregulation, liberalization, and privatization that swept the world since 1970.