Managing ecosystems for multiple benefits and stakeholders is a formidable challenge requiring diverse knowledge to be discovered, transmitted, and aggregated. Cost–benefit analysis (CBA) is advocated as a theoretically grounded decision-support tool, but in practice it frequently appears to exert little influence. To understand this puzzle, I consider ecosystem knowledge and CBA from both the demand and supply sides. I argue that all ecosystem knowledge is contestable, which restricts the influence of technocratic tools like CBA. On the demand side, democratic mechanisms shape decision makers’ motivations and incentives, but also provide a substitute for technocratic evidence. Supply-side factors limiting the influence of CBA include the scarcity of decision-pertinent evidence and the uncertain meaning and usefulness of CBA. Demand-side factors are resistant to change; but taking account of them, I suggest some supply-side reforms, arguing that CBA is best regarded not as a tool but as a venue where ecosystem knowledge is aggregated and contested.