The aim of project managers is to strike balance between the costs and the time resulting in the optimum project schedule.
An optimum project schedule implies the lowest possible cost and the associated optimum time for the project.
Each activity, in a project, consumes some resources and hence, has cost associated with it. In most cases, the cost of an activity will vary to some extent with the amount of time consumed by the activity.
The total cost of the project also depends upon the total duration of the project.