Sort users by meta_value_num

Add this code in functions.php.
Use orderby with meta_value_num and define meta_key in argument array.

add_action( 'pre_user_query', 'wps_pre_user_query' );

function wps_pre_user_query( &$query ) {
    global $wpdb;

    if ( isset( $query->query_vars['orderby'] ) && 'meta_value_num' == $query->query_vars['orderby'] )
        $query->query_orderby = str_replace( 'user_login', "$wpdb->usermeta.meta_value+0", $query->query_orderby );
}