Querying with WP query using meta key price

'meta_value_num' – Order by numeric meta value (available with Version 2.8). Also note that a 'meta_key=keyname' must also be present in the query. This value allows for numerical sorting as noted above in 'meta_value'.

Try to use like this :

 $args = array (
            'post_type'     => 'product',
            'post__in'      => $post_ids,
            'posts_per_page' => -1,
            'order' => 'ASC',
            'orderby' => 'meta_value_num',
            'meta_key' => 'tour_price',
            'meta_query' => array(
               array(
               'key'       => 'tour_price',
               'compare'   => '=',
               'type'      => 'NUMERIC',
               )
            ),
        );