To reproduce:
- Add calculated field
- Try to set its number format, by clicking the Number Format option in the context menu in the field list (see image)

Workaround:
 Private Sub RadPivotGrid1_CellFormatting(sender As Object, e As UI.PivotCellEventArgs) Handles RadPivotGrid1.CellFormatting
        If e.CellElement.Column.Name = "Commission" Then
            e.CellElement.Text = String.Format(System.Threading.Thread.CurrentThread.CurrentUICulture, "{0:C}", e.CellElement.Value)
        End If
    End Sub