In this section of your project management plan you will describe how cost and cost variances will be managed.  Of course, the costs you affix will be estimates and should provide an acceptable range of variance, the conditions of the estimate and any assumptions.  For example, you may estimate that a particular item or step will cost $xx.xx with a +/- 10% variance and the assumption that this estimate is only good for 90 days.  Your project management plan will include how you will measure and report your costs with the ultimate goal of completing the project within budget.