The concept of cost is an integral element of ecological theories, including optimal defense theory, resource availability theory, and growth-differentiation balance theory. Indeed most frameworks that attempt to explain within-plant patterns of secondary metabolites, as well as account for the evolution of induced defenses, presume that defenses are ‘costly’. One way in which investigators have sought to quantify the cost of secondary metabolites is to examine growth/defense tradeoffs, which are predicted to occur wherever resources cannot be simultaneously allocated to both growth and defense. However, emerging evidence suggests that these critical assumptions may not be valid for brown algal phlorotannins, compounds that occur throughout the division Phaeophyta and have served as analogs to vascular plant tannins in numerous tests of terrestrial-derived ecological theories in the marine environment. Here we present a model of phlorotannins as metabolites with both primary and secondary roles and argue that apparent trade-offs between algal growth and phlorotannin content are not a reliable indicator for establishing a cost of defense. We suggest the ecological theories which presume that defenses are costly because resources allocated to defense cannot also be allocated to other ‘primary’ functions are unlikely to accurately predict the striking variations in algal phlorotannin concentrations that are observed in nature.