My best month was around £120 (in February 2014) (~$200) through Amazon affiliate links & Admob. All within a very simple shopping app 'designed' for android 2.1 back in the day and updated just once in 2 years. I've since pulled it due to changes upstream breaking data so it became useless, I didn't find the time to really fix everything.
I have just launched (like a few days ago) http://photobrix.com - but that is yet to bring in a penny from the limited adverts - I'm likely going to add a higher end/more featureful interface to generate instructions that people will spend $3 - 5 or so on. Additionally I'm aiming on allowing users to order their own prints (rather than deal with the hassle of individual bricks).
I have just launched (like a few days ago) http://photobrix.com - but that is yet to bring in a penny from the limited adverts - I'm likely going to add a higher end/more featureful interface to generate instructions that people will spend $3 - 5 or so on. Additionally I'm aiming on allowing users to order their own prints (rather than deal with the hassle of individual bricks).