Cart Class - Update price?

[eluser]KazooSoft[/eluser]
Hey all,

I have a complicated product which basicly gets cheaper the more you buy... Many around. To do this im handling a If statment which will determing every time the cart is updated. It works perfectly for the add product, but not when updating.

Whats wrong with this?

Code:

$data = array(  
        'rowid' => $item[$i],
        'price'   => $cost,
        'qty'   => $qty[$i]                              
        );  
$this->cart->update($data);

Is it not possible to update the price?