This article demonstrates the Comprehensive Approach, by means of an example of an inter-basin transfer scheme with significant pumping costs associated with water transfers. Uncertainties about future water transfers and associated variable costs are provided by stochastic simulation modelling. The incremental approach is shown to be severely biased with respect to variable costs and it is shown that this bias leads to significantly different estimations of likely life-cycle project costs. Such differences conceivably lead to suboptimal decision-making.