Yep, you're right Allan. There's no way to know for sure.

Hypothetical calculation for the 2000 visitor/day goal I suggested:

Divide $500 by 30 days, leaves a goal of earning $16.67 per day.
Visitorship of 2000/day would require earning $8.34 revenue per thousand visitors.
3 ad units per page, 3 average pageviews per visitor = 9 ad units shown per visitor.
9000 ads shown per 1000 visitors that need to earn avg of $8.34 = $.93 CPM goal.

I can't say if this is possible or not with Adsense.