Costs

The amount of money that is spent to produce something of value. In a task plan, a cost is the amount of money that is spent while finishing a task. For example, a resource's hourly billing rate or the cost of renting a tractor (a material resource) for a day. A cost can be associated with a task, a resource, an assignment, or a task plan.

There are three types of costs: fixed, per-use, and rate.

A fixed cost is one that does not change regardless of a task's duration or how much actual work is completed.
A per-use cost is one that is applied each time a resource is used. When a work resource is associated with a per-use cost, that cost is applied each time that resource is assigned work in a task plan. When a material resource is associated with a per-use cost, that cost is applied once.
A rate cost is one that is applied based on a duration (typically, per-hour). A rate cost is most often associated with a work resource and it is used to represent the units worked (typically hours) against a task in a task plan. There are two types of rates: regular and overtime.

You can use a baseline to compare original costs to actual costs over the life of a task plan by comparing the variance between the original cost values and the actual cost values.