Allows to override a price of an item in the shopping cart. 
The overridden price will be correctly processed by the discount and tax engines.
Usage example:
public function subscribeEvents()
{
  Backend::$events->addEvent('shop:onGetCartItemPrice', $this, 'get_cart_price');
}
public function get_cart_price($cart_item)
{
  if ($cart_item->product->sku == '26268880')
    return 10;
}