Developments in fusion energy technology and the aspiration to build and run commercial fusion energy power plants has seen the commencement of numerous publicly and privately funded projects in recent years. Megaprojects, like fusion power plants, by their very nature are inherently complex and risky, therefore providing a robust cost estimate in early stages is challenging. In the fusion sector, this is amplified by the fact that very little data exists on which to base an estimate due to the novel nature of the materials and technologies involved. A common phenomenon during the estimating phase of a new project is the concept of Optimism Bias, where under-estimation of time, cost and risk can cause impacts on the baseline cost and schedule, leading to significant over-runs during the project lifetime. Reference Class Forecasting is a method used to mitigate against these factors, based on using actual performance data in a reference class of comparable projects to improve forecasting accuracy. This article will discuss Reference Class Forecasting, how it has been used in recent megaprojects, and how it is intended to be used in the STEP (Spherical Tokamak for Energy Production) Programme to provide a full programme cost model for a Prototype Fusion Energy Plant.