The extent to which conservation is feasible is constrained by budgets and the financial sacrifice stakeholders are willing to bear. Therefore a possible objective for conserving a species is to minimise the cost of achieving that stated aim. For example, if a minimum viable population (MVP) of a species is to be conserved, the size and type of habitats reserved for this could be selected to minimise cost. This requires consideration of the comparative (relative) opportunity costs of reserving different land types for conservation. A general model is developed to demonstrate this and is applied to the case of the orangutan. In the ecological literature, recommendations for reserving different types of land for conservation have been based on comparisons of either the absolute economic returns they generate if converted to commercial use or on differences in the density of a species they support. These approaches are shown to be deficient because they ignore relative trade-offs between species population and economic conversion gains at alternative sites. The proposed model for orangutan conservation shows that where land conversion may be impending, the selection of habitats (peat forests or dryland forests or combinations of both) for securing an MVP may in fact be different when comparative costs are factored in than if only absolute values are considered.