Im reading MS Project Inside out and I cud find this information for you in chapter 8.
I have not used this feature. But this information wuill surely help you out.
Go to res sheet
Open Res Information Dialog box
Click on Cost
Under A(Default Tab) you can enter Std rate and Overtime rate.
To specify a change in rate after a certain period of time, click in the next blank Effective Date field and enter the date the change is to take effect. Enter the cost changes as applicable in the Standard Rate, Overtime Rate, and Per Use Cost fields
To specify different costs based on different types of activities, enter the different costs in a different tab, such as B or C.
Click the B tab, for example, and enter the standard rate, overtime rate, and peruse cost for the other activity as applicable. When you assign this resource to a task that uses the different rates, you can specify them with the assignment.
If a percentage rate change goes into effect on a certain date, you can have Microsoft Project calculate the new rate for you. Enter the date in the Effective Date field. Then, in the Standard Rate, Overtime Rate, or Per Use Cost fields, enter the percentage change, for example, +10% or -15%. The actual rate representing that change is immediately calculated and entered in the field.
Cost Rate Table A for resources is applied to the resource's assignments by default and is what shows on the Resource Sheet. If you define a different cost rate table for another category of work, you can specify which cost rate table is to be used at the assignment level To do this, follow these steps:
In a task view such as the Gantt Chart view, use the Assign Resources dialog box to assign the resource to the task.
Click View, Task Usage or View, Resource Usage to switch to an assignment view.
Click the assignment that needs a different cost rate table applied and then click Assignment Information on the Standard toolbar.
If necessary, click the General or Tracking tab.
In the Cost Rate Table list, click the cost rate table you want to apply to this assignment
In the Assignment Information dialog box, select which cost rate table should be used for this assignment.
There is no way to enter a description for the different cost rate tables to explain what each one is to be used for. Because of this, it's a good idea to enter a resource note for any resource that uses multiple cost rate tables. Simply click the Notes tab in the Resource Information dialog box and enter the note
