Meta box does not appear in a custom post type.

I am trying to create a custom post type for products of a company with a text box to add the price. But no matter what I try, the meta box does not show up. Could any one help me? The code I used is,

class vp_serivce_post{
    public function __construct() {
        $this->ser_registar();
        $this->price_box();
        add_theme_support('post-thumbnails');
//        set_post_thumbnail_size( 80, 80, true );
    }

    public function ser_registar(){

        $args = array(
            'labels' => array(
                'name' => 'Products',
                'singular_name' => 'Product',
                'add_new' => 'Add New Product',
                'add_new_item' => 'Add New Product',
                'edit_item' => 'Edit Product',
                'new_item' => 'New Product',
                'all_items' => 'All Products',
                'view_item' => 'View Product',
                'search_items' => 'Search Products',
                'not_found' => 'No Product Found',
                'not_found_in_trash' => 'No Product Found in Trash',
                'menu_name' => 'Products'
            ),
            query_var => 'products',
            rewrite => array(
                slug => 'products/'
            ),
            'public' => true,
            //'menu_position' => 25,
            'supports' => array(
                'title',
                'thumbnail',
                'editor'
            )
        );
        register_post_type('services',$args);
    }

    public function price_box(){
        add_action('add_meta_boxes', 'add_price_box');

        function add_price_box(){
            add_meta_box('price_id', 'Price', 'price_add', 'vp_price');

            function price_add($post){
?>
<label for="vp_price">Price: </label>
<input type="text" class="widefat" name="vp_price" id="vp_price" vlaue="<?php echo esc_attr($length); ?>" />
<?php
            }
        }
    }
}

add_action('init', 'post_invok');

function post_invok(){
    new vp_serivce_post();
}