You do not have to add the FormulaTag until you know what you want to add. In your case, you would add it in your Add Button handler.
Are Rate and Formula loaded from, and saved to, your DataBase? If so, you could try setting the GridBoundColumn.StyleInfo.CellType for Rate column to be "FormulaCell".
If they are not columns in your DataBase, then are they to be considered unbound columns that will not be persisted anywhere?
Another option you could use is to use Expression columns directly in your DataTable. This way, the DataTable will handle the calculations, and the grid will just display the columns like they were just any other column.
Anyway, here is a sample doing what you described, adding a new row when you click a button.
forum12973_2054.zip