In the long-term development of the electric power system, system operators should consider the socio-economic balance between grid investment costs and security of supply, including the risk of power supply interruptions. Cost-benefit analyses conducted for this purpose are associated with many uncertainties but have traditionally focused on the expected value of the net socio-economic benefits of risk-reducing measures. This article focuses on the large uncertainties that are associated with the possible occurrence of high-impact low-probability interruption events (HILP events). The objective is to quantify and visualize the implications of uncertainties due to HILP events in the context of power system development. More specifically, this article describes a methodology accounting for uncertainties in socio-economic cost-benefit analysis of measures for reducing the risk of HILP events. The methodology accounts for the contributions of both aleatory and epistemic uncertainties and comprises a hybrid probabilistic-possibilistic uncertainty analysis method. Applying the methodology to a real case involving a grid investment decision, it is demonstrated how it provides additional insight compared to conventional cost-benefit analyses considering expected values where uncertainties are not accounted for explicitly. It is furthermore discussed how these results can help to better inform grid development decisions.