Change subtotal with given price & quantity (custom attribute) - magento

Question

My price is for example 10,00 €
this price is for 100 g
the customer can add any g in the quantity field for example 300
magento has now a subtotal of 3000, its ok but not for my needs here.
i need to do:
if price & quantity is set, get subtotal / price quantity, set new subtotal
where can i put my modifications for this?
Thank you very much!
Dennis
Edit
and the third try of observer (not working atm, no error, nothing happens):
class Xyz_Catalog_Model_Price_Observer
{
public function __construct()
{
}
public function apply_quantity_order($observer)
{
$order = $observer->getEvent()->getOrder();
$pricequantity = $order->getPricequantity();
if($pricequantity != ''){
$old_sub_total = $order->getTotals();
$new_sub_total = $old_sub_total / 100;
$order->setTotals($new_sub_total);
} else {}
return $this;
}
public function apply_quantity_quote($observer)
{
$quote = $observer->getEvent()->getQuote();
$pricequantity = $quote->getPricequantity();
if($pricequantity != ''){
$old_sub_total = $quote->getTotals();
$new_sub_total = $old_sub_total / 100;
$quote->setTotals($new_sub_total);
} else {}
return $this;
}
}
XML:
<?xml version="1.0"?>
<config>
<global>
<models>
<xyzcatalog>
<class>Xyz_Catalog_Model</class>
</xyzcatalog>
</models>
<events>
<sales_order_save_after>
<observers>
<xyz_catalog_price_observer>
<class>Xyz_Catalog_Model_Price_Observer</class>
<method>apply_quantity_order</method>
</xyz_catalog_price_observer>
</observers>
</sales_order_save_after>
<sales_quote_save_after>
<observers>
<xyz_catalog_price_observer>
<class>Xyz_Catalog_Model_Price_Observer</class>
<method>apply_quantity_quote</method>
</xyz_catalog_price_observer>
</observers>
</sales_quote_save_after>
</events>
</global>
</config>

Answer 1

Rather than overriding sub-total calculation function, I suggest to try events - sales_quote_save_after and sales_order_save_after.
You can get quote and sales in observer method by
$observer->getEvent()->getOrder() //for order
$observer->getEvent()->getQuote() //for quote
Then modify the subtotal accordingly.
Edit: It might be just hint how can you modify sub total.
Edit2: You have to add event observer in your config as shown:
<sales_order_save_after>
<observers>
<yourext>
<class>yourext/observer</class>
<method>observerMethod</method>
</yourext>
</observers>
</sales_order_save_after>
For detail, have look on Customize Magento using Event/Observer

Answer 2

Take a look # Programmatically add product to cart with price change
public function applyDiscount(Varien_Event_Observer $observer)
{
/* #var $item Mage_Sales_Model_Quote_Item */
$item = $observer->getQuoteItem();
if ($item->getParentItem()) {
$item = $item->getParentItem();
}
// calc special price
$percentDiscount = 5;
$specialPrice = $item->getOriginalPrice() - $percentDiscount;
// Make sure we don't have a negative
if ($specialPrice > 0) {
$item->setCustomPrice($specialPrice);
$item->setOriginalCustomPrice($specialPrice);
$item->getProduct()->setIsSuperMode(true);
}
}