Adding costs to P6

Resource Loaded Costs (common, but lot to manage)

• You can assign costs to resources. So when you assign a resource to an activity, the cost rolls up as that resource is used (hours x rate).
• You can change their rate in resource dictionary.
• Add resources on fly once they in the dictionary.
• When resources are typically added, their rate is just multiplied by the duration. 

Expenses added to activities (somewhat common)

• You add an expense to an activity and reduce it over the course of the duration of the activity. Very manual.
• You can front-load, back-load, or do uniform spread over activity. 

Also good videos on YouTube that explain it.
https://www.youtube.com/results?search_query=adding+costs+in+p6