Since pay rates can and do change, you'll want to set this up so that a new payrate won't affect values in older records that referred to the payrate then in effect.
Define a pay rate field in your related employee table. Define a number field in your timesheet field and select the Looked up Value field option for it on the Auto-Enter tab. Set this option to copy the pay rate from the employee table. Assuming the two tables are linked by EmployeeID, selecting an EmployeeID value in the timesheet table will automatically copy the current payrate to the timesheet record.
To keep this field from being edited on the timesheet layout, select it while in layout mode and use Inspector | Data Tab | Behavior to clear the "Browse mode" check box. Now the field cannot be entered and edited on the timesheet layout.
To keep the field in the employee table from being edited by an employee, keep the field on a layout that only users with sufficient access permission (set this up in Manage Security) can access.