This has been discussed before, and I can't remember now why we did not include it. The code that prepares the values that you can use in the email is in the quiz_send_notification_emails function in mod/quiz/locallib.php. I think if you add:
$a->mark = quiz_format_grade($quiz, $attempt->sumgrades);
$a->maxmark = quiz_format_grade($quiz, $quiz->sumgrades);
$a->grade = quiz_rescale_grade($attempt->sumgrades, $quiz);
$a->maxgrade = quiz_format_grade($quiz, $quiz->grade);
$a->percent = format_float($attempt->sumgrades / $quiz->sumgrades * 100, 0);
there, it will do what you want. I guess we should consider including this code in the official version. Can anyone find the previous discussion on this topic?
(One argument I think I remember is that if you just email the students their score, then the probably won't bother to go online and look at the feedback, which might teach them something, but just because that might happen, we shoudl not make it impossible for people to customise the email to include the score.)