The Developer of an app writes the description you see in the App Store. If they don't make a convincing case for you to easily determine what features are free and what you will gain from a specific in-app purchase, then it is a failure on their part and you should move on to one that states their case clearly.
I also recommend reading the reviews for making a determination before download. If you download an app and the in-app purchases are not clear or you feel they are unfair, then write a review on the app to let others know your experience.
Most every app that offers in-app purchases are free to download, so there is no risk of losing any money, because that would require you to double click the side button with the amount showing on the screen, followed by authentication with FaceID/TouchID or Passcode.