With a SUBSCRIPTION based business model for a mobile application, you have to tie in MORE than just the CPA but the cost to acquire users who take specific actions.
Businesses with a mobile app, may have to prove that your app is usable and functioning to validate that it is worth advertisers investing, rather than an app that is solely reliant on sales of their own products and services.
For example…
If you have an app that relies on affiliate marketing to get users to ask questions inside of the app. It is up to the partner to close those deals based on how they respond. It would be your responsibility as the mobile app to generate the traffic and create a user experience that is beneficial to your partners.
So therefore your business model is to inspire users to take STEPS toward turning a profit for your partners. This would be in the form of positioning interactions with specific content of affiliate partners and users asking questions inside of the app.
Therefore success isn’t defined by a sale, but how well users will engage with affiliate content.
With the proper data points set up you can do CRO (Conversion Rate Optimization) to get more users who may ask that SAME question(s).
Have you considered this?