Conservation initiatives to protect valued species communities in human-dominated landscapes face challenges linked to their potential costs. Conservation covenants on private land may represent a cost-effective alternative to land purchase, although many questions on the long-term monitoring and enforcement costs of covenants and the risk of violation or legal challenges remain unquantified. We explore the cost-effectiveness of conservation covenants, defined here as the fraction of the high-biodiversity landscape potentially protected via investment in covenants versus land purchase. We show that covenant violation and dispute rates substantially affect the estimated long-term cost-effectiveness of a covenant versus land purchase strategy. Our results suggest the long-term cost-effectiveness of conservation covenants may outperform land purchase as a strategy to protect biodiversity as long as disputes and legal challenges are low, but point to a critical need for monitoring data to reduce uncertainty and maximize conservation investment cost-effectiveness.