Being a novel technology, estimating costs for fusion power plants comes with large uncertainties. Cost uncertainties in prototypes arise from various sources and reduce with programme maturity (including the selection of a site), technical and design maturity, maturity of the commercial strategy (e.g., make vs. buy decisions, partnering decisions) and the creation of a fusion relevant supply chain. In the early design phases, where not much detail is fixed, cost is typically evaluated via analogy/comparator to existing technologies which, due to their nature, are typically are associated with high uncertainties. Early cost estimates for fusion power plants are broken down using a top-level cost break down structure. Each component of the structure has been estimated using a different technique and making different assumptions.  Therefore, each subcomponent of the cost breakdown structure has a different uncertainty attached to it. These can be propagated to the higher levels of the structure using Monte-Carlo techniques. We will also explore the various aspects that drive cost uncertainties on such a fusion project, looking into the effect of programme scope and Technology Readiness Levels. The overall motivation being to illustrate the importance of uncertainty definition within fusion prototype/demonstration powerplants and their impact on programme costs.