Magento update subtotal item -

i can't update subtotal item in cart. created module observe checkout_cart_product_add_after can subtotal price item:

$subtotal = mage::getsingleton('checkout/cart')->getquote()->getsubtotal(); 

but can't update this, exmaple:

$subtotal = $subtotal + 100; mage::getsingleton('checkout/session')->getquote()->setsubtotal($subtotal); mage::getsingleton('checkout/cart')->getquote()->setsubtotal($subtotal); mage::getsingleton('checkout/session')->getquote()->save(); mage::getsingleton('checkout/cart')->getquote()->save(); 

edit

if run in observer print_r($subtotal); exit; correct updated subtotal. in cart page have still orginal subtotal without change.

edit 2 i'm trying run modifysubtotal function sales_quote_collect_totals_after event, can't see on cart page updated subtotal price. below code of modifysubtotal observer.php:

public function modifysubtotal(varien_event_observer $observer)     {      $session = mage::getsingleton('checkout/session');     $quote=$observer->getquote();     $subtotal = $quote->getbasesubtotal();     $subtotal = $subtotal +123;     $quote->setbasesubtotal($subtotal);      $quote->save();     $subtotal2 = $quote->getbasesubtotal();     //print_r($subtotal2);exit;  } 

i'll grateful tips , help.

you should observe sales_quote_collect_totals_after.