Yes, Commerce Module does store product amount of past order... You can retrieve using below details...


To get full details of order

    $order = commerce_order_load($order_id);
    $view_mode = "administrator";
    print entity_view('commerce_order', array($order->order_id => $order), $view_mode, NULL, TRUE);


To be more specific

      $order = commerce_order_load($order_id);
      foreach ($order->commerce_line_items['und'] as $line) {
        $line_item = commerce_line_item_load($line['line_item_id']);
        dpm($line_item);
      }

There is no chance it gets new price instead of old price since it gets tracked in `field_data_commerce_unit_price` table in database ... All you need to do is divide by 100 if decimal places are 2 in order line item amount...

![enter image description here][1]


  [1]: https://i.sstatic.net/bbbns.png