I think its low by comparison to other apps that generate significantly more and/or if your expectations are relatively high. But his expectations were to get ~$100/month so I'm sure he is ecstatic. And besides, an extra $36,000 a year in relatively passive income isn't too shabby.

Getting to the top of a category in the app store is a metric based on installs. Ad revenue is based on number of times the app is actually used.

Developing an app that thousands of people install but don't actually use often won't make you especially rich.

Agreed. But I still tought the top category app should drive so many installs that even users launching the app just once should generate more than that. Looks like the the business category is driving less installs than other more popular ones, like games or photo apps...