Grassland conversion into croplands in the Prairie Pothole Region of the United States is a persistent hurdle toward mitigating climate change. Several carbon offset markets have been designed to reward landowners for keeping lands in their native state when incentives to convert are high. We explore the role of a critical determinant in such programs: the additionality threshold. This factor, if appropriately selected and applied, reduces the participation of landowners that would choose to enroll in the program but would not have converted their land under business-as-usual conditions. Using a simple model relating land quality and land use to economic rents, we simulate potential avoided grassland conversion offset market participation across a range of cropland over pasture rent difference threshold (RDT) values. We find mitigation potential and simulated program costs are widely variable depending on this parameter and assume carbon prices: across the five states studied, the full range is 0.41 tCO₂e • yr^–1 (0.2 RDT, $10 •$t^–1 carbon price) to 4.6 million tCO₂e • yr^–1 (1.2 RDT, $40 tCO₂e • t^–1 carbon price), assuming average land use change emissions values for pastureland in the region. Total program costs for these offsets also exhibit a wide range, spanning $2–$120 million • yr^–1 depending on parameterization. Results across the full range of RDTs (0.2–2) demonstrate a tendency toward higher RDTs for achieving high levels of avoided emissions, with cost efficiency being maximized in the 1.4–1.8 range for RDTs. A state-level breakdown of results demonstrate the importance of modeling economic trends in land use and setting region-specific additionality thresholds for avoided grassland conversion offsets. Although our study is specific to grassland conversion in one region of the United States, similar offset markets exist elsewhere, where additionality concerns are paramount. We believe our framework can be useful in improving protocol design.