Cost (in terms of resources) of changing the SoftwareDesign in mid or late phase of a project.

Most traditional (non-AgileSoftwareDevelopment) process presents the change cost curve as an ExponentialCostCurve, hence BigDesignUpFront is encouraged and IncrementalDevelopment is discouraged.

XP Proposes a flattened cost curve by applying EnablingPracticesOfXp to achieve an EvolutionaryDesign.

The change in cost curve essentially means the transition from the flattened cost curve to the ExponentialCostCurve.