I made about $1,500 per month with this App for about a year [1]. Over time the downloads went down and so did the revenue. (It came down to about $300 per month).
This website, which I own started it all [2]. The website was always free and the app was an revenue generating up-sell for the website.
In Dec 2014, I wanted to sell the Website, but all the sites like Flippa wanted Adwords Revenue, so I put Adwords on the site...to show some revenue and try and sell the site.
Adwords revenue immediately jumped to $2K a month, so I decided to make the app free again, since it was only making $300 and I would rather get it in as many people's hands/pockets as possible.
Comment if you have any other questions.
[1]https://itunes.apple.com/us/app/7-min-workout/id657369636?mt...