Objective The objective of this study was to demonstrate how decision‐analytic modelling can help to determine circumstances under which surgery may become cost‐effective, using septoplasty as an example. Design We developed a decision‐analytic model comparing septoplasty to non‐surgical management for nasal obstruction in adults with a deviated septum. Based on the estimated cost difference between both treatments, we calculated the minimal (a) gain in quality‐adjusted life‐years, or (b) reduction in productivity losses needed for septoplasty to be cost‐effective. Input was derived from literature and publicly available data sources. The time horizon of our model was one year, and the willingness‐to‐pay per quality‐adjusted life‐year was €20 000, in accordance with current guidelines. Results The cost difference between septoplasty and non‐surgical management for nasal obstruction due to a deviated nasal septum was €2227 per patient from a healthcare perspective (including direct healthcare costs) and €3288 per patient from an extended perspective (additionally including travel expenses and productivity losses due to poor health). In comparison with non‐surgical management, septoplasty needed to gain 0.11 to 0.16 QALYs or save 13 sick days for nasal obstruction. The longer septoplasty's effect lasts, the more time it will have to compensate its extra costs. Conclusion This study shows that the known cost difference between treatments can be used as the starting point to determine beneficial effects needed for cost‐effectiveness of surgical interventions. The effect required by septoplasty from a healthcare perspective seems potentially achievable, meaning that it would be useful to perform an RCT assessing the actual benefits of septoplasty.