Addressing problem with Smashing Magazine's Custom Post Type

function product_price_box_content( $post ) {
    wp_nonce_field( plugin_basename( __FILE__ ), 'product_price_box_content_nonce' );
    $meta_values = get_post_meta($post->ID, 'product_price', true);

    echo '<label for="product_price"></label>';
    if($meta_values != '') {
        echo '<input type="text" id="product_price" name="product_price" placeholder="enter a price" value="' . $meta_values . '"/>';
    } else {
        echo '<input type="text" id="product_price" name="product_price" placeholder="enter a price"/>';
    }
}