Not all apps are worth $5/month, but this particular one that people apparently love enough to spend hours per day on certainly seems like it should be. A slightly nicer calculator app that you use twice a month? $0.99 is fine. A professional productivity app that saves a high-value worker hours of time? Way more than $5/month.
It's ok if not everyone can afford an app.
> Why should the app developer pay anything at all?
The app developer is just a proxy for the users, who are using Reddit without seeing any of Reddit's ads.