Explanation
A cost management plan is a document that defines how the project will be estimated, budgeted, and how changes to cost will be managed. It is a subsidiary plan of the project management plan and it describes the processes, tools, techniques, and methodologies that will be used to plan, execute, monitor, and control the project costs. A cost management plan also specifies the level of accuracy, units of measure, organizational procedures, reporting formats, and thresholds for cost variances and corrective actions. References:
Section 7: Project Cost Management - Q & As for the PMBOK Guide Sixth ..., section 7.1, page 1 Project Cost Estimation: How to Estimate Project Cost - ProjectManager, under What Is a Project Budget?
Project cost management basics - Clockify, under Step #1: Plan the resources