I don't think that there is an average income from google adsense. There is a huge gap on how much people are earning. I would say that the majority of the publishers earn less than $100/month, while they are some top publishers that earn in excess of $10,000/month.

You could start by setting some goals for yourself. For example, you should start with a goal of $3/day, then when you pass this goal, try $10/day, and shoot for $100/day later.

Best of luck.