I am currently charging a one time price for my app https://www.ghostnoteapp.com it's doing pretty well but it's also hard work. I am currently working on a subscription version of my product mostly used for team annotation of websites, something a lot of people have been asking me about. And I really want to move towards recurring revenue as there is much more to work with.

The best trick I can give you is start with a really low price then slowly raise it until downloads slow down but your revenue is intact from your start price.

I don't believe you need to ever introduce ads unless your product provides no real value.