It’s not $99 for the app though, it’s $20 for sync ad-free, which is reasonable for a crazy polished app like this one. The $99 gets you a tiny bit more functionality they provide extra based around a subscription model because recurring revenue makes sure you always have the funds to further your project. I don’t necessarily agree with this sentiment, but I also don’t need any of those extra features.
Also, sync is just the beginning. Many more apps will follow. If they still think that price is reasonable, then so be it. But $20 is very fair IMO.
It doesn’t justify it IMO, but the features are not the point. It’s about supporting the development for a (currently) smaller userbase.
Anyway, here’s a screenshot