Many threatened species in human-dominated systems are managed through conservation programs. Such programs are sometimes designed based on intuition or short-term results rather than assessing their long-term biological and economic sustainability. The current conservation program for Montagu's harriers Circus pygargus, a ground-nesting bird of prey, in Lleida (Catalonia, NE Spain) aims to protect nests located in farmlands by promoting crop harvest delay around the nest and compensating farmers for their economic loss. This program has been flagged as a 'conservation trap' as its costs have been increasing over time, possibly compromising the long-term sustainability of the program and associated consequences to the local harrier population. In the present work, population viability analyses (PVA) were used to find a conservation management scenario that decreases the risk of the conservation trap, or at least minimizes the medium-term expenditure on conservation. PVA simulations suggest that the current nest protection program is financially unsustainable at the medium-term. Cost-effectiveness analyses suggest that it would be impossible to fully avoid the conservation trap if the conservation goal is to maintain Lleida's current population size. Alternative management scenarios that minimize the medium-term expenditure of scarce conservation funds are presented. The results suggest that selecting a conservation program based only on short-term biological or cost-effective targets might not be the most appropriate, and demonstrate the relevance of having clear medium-term conservation targets.