Project cost via timesheets

HI All,
I am a new user of ERPNext and I am trying, without any success, to use the projects module. My goal is to create the timesheets to calculate the time an employee worked on the project and the related cost.
I created the project, activities and timesheet of the employee. In the project the hours coming from the timesheet are correctly calculated but I can’t calculate costs. I cannot assign hourly costs to activities because employees with different costs are involved in the same activity.
I have specified for the employee:

  1. The Salary Structure Assignment
  2. The basic earning;
    What’s wrong, does anyone have any suggestions?
    thanks in advance