How to Change the price before adding to cart in magento?

How to Change the product price while adding to cart Magento, Example :

suppose I am trying to add 1 product of price $10, I want to show the price with $10*5 = $50, when I add 2 products I want to show $10*10 = $100. so its common multiplier is 5.(multiples of x).

Create Observer.php file in app/community/Custom_Module/Modulename/Model copy the below code inside that file.

class Custom_Module_Modulename_Model_Observer
{
    public function _construct()
    {
    }

    public function getNewPrice()
    {

        $login  = Mage::getSingleton('customer/session')->isLoggedIn();
        $roleId = Mage::getSingleton('customer/session')->getCustomerGroupId();
        $userrole   = Mage::getSingleton('customer/group')->load($roleId)->getData('customer_group_code');
        $userrole   = strtolower($userrole);
        $quote = Mage::getSingleton('checkout/session')->getQuote();
        $cartItems = $quote->getAllVisibleItems();
        foreach ($cartItems as $item) {
            $productId = $item->getProductId();
            $product = Mage::getModel('catalog/product')->load($productId);
        }
        $batch_qty = $product->getBatchQty();
        $actualPrice = $product->getPrice();
        $specialPrice = $product->getFinalPrice();
        if (isset($batch_qty) && $userrole=="retailer") {
            if (isset($specialPrice)) {
                $newprice = $specialPrice*$batch_qty;
            } else {

                $newprice = $actualPrice*$batch_qty;
            }

        } else {
            $newprice= $actualPrice;
        }

        return $newprice;
    }

    public function updatePrice($observer)
    {
        $event = $observer->getEvent();
        $product = $event->getProduct();
        $quote_item = $event->getQuoteItem();
        $new_price = $this->getNewPrice();
        $quote_item->setOriginalCustomPrice($new_price);
        //$quote_item->save();
        $quote_item->getQuote()->save();
        //Mage::getSingleton('checkout/cart')->save();
    }

}

Copy the below code and paste it your app/community/Custom_Module/Modulename/etc/config.xml inside tag

> <events>            
>           <sales_quote_add_item>
>               <observers>
>                  <Custom_Module_Modulename_model_observer>
>                     <type>singleton</type>
>                     <class>Custom_Module_Modulename_Model_Observer</class>
>                     <method>updatePrice</method>
>                  </Custom_Module_Modulename_model_observer>
>              </observers>
>           </sales_quote_add_item>
>       </events>

This is working fine for me.