To restrict products having price zero from shopping we need to create a observer on
checkout_cart_product_add_after event such that on every add to cart it will check the
product price and then add to cart.

Create a module and in etc/config.xml define the observer under event node.

Here Our module name is zeroprice under Letsknowit namespace.

File path : app/code/local/Letsknowit/zeroprice/etc/config.xml

                    singleton

                    zeroprice/observer

                    checkProductPrice

Now in Model folder we need to create observer with function name checkProductPrice.

File path : app/code/local/Letsknowit/zeroprice/Model/Observer.php

Class Letsknowit_Zeroprice_Model_Observer {

      public function checkProductPrice(Varien_Event_Observer $observer) {

        if ($observer->getProduct()->getFinalPrice() == 0) {

            Mage::getSingleton('core/session')->addError('Sorry, you cannot purchase "'.$observer->getProduct()->getName().'" product.Write to us if any query!'));

            $url = Mage::getModel('core/url')->getUrl('checkout/cart/index');

            Mage::app()->getResponse()->setRedirect($url);

            Mage::app()->getResponse()->sendResponse();

            exit();

        }

    }

}

?>