I have made a very little with my two to three blogs...but the thing is that i do not have to lift my finger to earn that money and i am very much sure if i work on them even an hour or two daily..i can earn up to $20 daily.which i think is not bad.
Across all of my 20+ sites, my personal account does just under $20 per month. On the other hand, my blog network of 1000s of sites gets quite a bit more.