How do you know what an app costs to “buy” and what monthly fees apply

Look at the app in the App Store. If the app itself has a cost this will show to the right of the name (if it’s free it says “Get” instead).

If the app runs on a paid subscription model or has other enhancements you can pay for, these will all be shown in In-App Purchases section near the bottom of the page on the App Store. They will probably be detailed in the app itself as well.