system->configuration->sales section ->checkout->shopping cart->After Adding a Product Redirect to Shopping Cart=no

mage->checkout->controllers->cartcontroller-> you need to work on addAction method (you can directly give $url=home url or product list url.and you need to override cartcontroller).

if (!$this->_getSession()->getNoCartRedirect(true)) {
                if (!$cart->getQuote()->getHasError()) {
                    $message = $this->__('%s was added to your shopping cart.', Mage::helper('core')->escapeHtml($product->getName()));
                    $this->_getSession()->addSuccess($message);
                }
                $this->_goBack();
            }

........... ....

$url = $this->_getSession()->getRedirectUrl(true);
            if ($url) {
                $this->getResponse()->setRedirect($url);
            } else {
                $this->_redirectReferer(Mage::helper('checkout/cart')->getCartUrl());
            }

There are many things wrong here and I feel compelled to do a code review with you. See the end for the correct approach.

Code review:

This line is loading a product with an undefined $sku var:

$product = Mage::getModel('catalog/product')->getCollection()
    ->addAttributeToFilter('sku', $sku)
    ->addAttributeToSelect('*')
    ->getFirstItem();

Now you're reloading the $product var with an undefined value, returns a blank model or null.

$product->load($product_id);

Getting the cart model (but not necessarily mine from my customer session):

$cart = Mage::getSingleton('checkout/cart');

Wait - now we're loading another instance of a blank object:

$productInstance = Mage::getModel('catalog/product')->load($product_id);

Clearing the brand new cart:

$cart->truncate();

Saving our empty cart:

$cart->save();

Getting all the items in our empty cart, clearing them, and saving again:

$cart->getItems()->clear()->save();

The right way:

All in all this code makes no sense. Some Google searching turns up the right way to add a product to the current user's cart with the correct price attached:

$quote = Mage::getSingleton('checkout/session')->getQuote();
$quote->addProduct($product, $qty);

$quote->collectTotals()->save();

Where $product and $qty are both defined, and $product is an instance of Mage_Catalog_Model_Product and $qty is an integer.