Try this:

add_filter(\'posts_where\', \'unquote_numeric_meta_value\', 10, 2);

function unquote_numeric_meta_value($where, $args) {

    $value = isset($args->query_vars[\'meta_value\']) ? $args->query_vars[\'meta_value\'] : false;

    if(is_numeric($value))
        $where = str_replace("\'{$value}\'", $value, $where);

    return $where;
}