$args = array(
            'post_type'         => 'listing',
            'posts_per_page'    => 15,
            'paged'             => $paged,
            'meta_key'          => $meta_prefix . 'price',
            'orderby'           => 'meta_value_num',
            'order'             => 'ASC'
        );
$listings = new WP_Query($args);