Harvard University published an example of a simple Cost management plan.
You definitely need to check this resource as its really professional and presents important project management activities and processes related to the cost management plan.
Contents of the publication:
1 The project manager and the Cost management plan
2 Cost management approach
2.1 Measuring project costs
3 Reporting form
4 Corrective actions
Read the full article here: https://cyber.harvard.edu/difficultproblems/Cost_management_plan