I don't think there is an average hubber, so It would be impossible to know what his or her score would be.
Actually, the hubscore is just a very general way to see if you might be improving. I don't think it should be used to compare one person to another, or to find out if you are 'average'. It might be an indication of your personal progress, but (as has been said hundreds of times) it is really not of great importance.
Being under 75, indicates you should be improving something . Scores are not necessarily related to earnings.