This is the wrong question to ask its like asking how much revenue is good. The more the better.

In general i would place a value of $.25 to $.75 in potential revenue on single MAU on average. However this varies based on the 3 levers which are: # of MAU, length of session, and frequency. It also varies on the type of monetization and app type ie in app purchase vs ads, games vs utilities. etc.

Another interesting thing we're seeing on apptopia now is people buying up android apps with large install base, but low engagement i.e. utilities. As it turns out you can monetize the apps with certain ad networks that install some addons to the phone and display ads outside the app or through push notification. Some examples: Pingjam for caller id ads, Airpush for push notification ads,StartApp and many more. At some point Google will catch up and block these networks but as of right now its a wild wild west out there....

hope this helps.