Wildlife conservation is severely limited by funding. Therefore, to maximize biodiversity outcomes, assessing financial costs of interventions is as important as assessing effectiveness. We reviewed the reporting of costs in studies testing the effectiveness of conservation interventions: 13.3% of studies provided numeric costs and 8.7% reported total costs. Even fewer studies broke down these totals into constituent costs, making it difficult to assess the relevance of costs to different contexts. Cost reporting differed between continents and the taxa/habitats targeted by interventions, with higher cost reporting in parts of the Global South. A further analysis of data focussed on mammals, identified that interventions related to agriculture, invasive species, transport, and residential development reported costs more frequently. We identify opportunities for conservationists to improve future practice through (i) encouraging systematic reporting and collation of intervention costs; (ii) using economic evaluation tools; and (iii) increasing understanding and skills in finance and economics.