I think that the sky is the limit with google ads. The more sites you have the more money you make, the more people visit these sites even more money you make... no limit.
Check out this thread, the guy earns over $9000 per month!! http://forums.digitalpoint.com/showthread.php?t=245061
Burta spelled it out pretty well a few posts back.. around 10,000 visitors per day should/can make you around $100/day with adsense... Properly monetize the site even more with affiliates, donations, and what-not, and you might even see a few $600 days with that same traffic ..and ohh what exciting days those can be!
I have a site thats 40K-ish, doesn't make crap, 7-8 dollars a day.. and that's with Tribal. Google? Maybe a dollar sometimes :/