I do the same - if an app is "free" but has In-app purchases, I'll simply avoid it, even if I'd be willing to pay some price for it if it works. I don't have time to download every app, open it, and navigate to its pricing page to see how much it costs.
You should be able to see the name, description, and price of all the in-app purchases from the App Store.
I’m not sure about the Mac OS side, but at least on iOS there’s a specific entry in the information section for in-app purchases that can be expanded into exactly that pricing list.
But it's always listed as something like "Premium Features" or "Monthly Subscription." Those descriptions don't tell what I'm getting. So then it's usually download the app, try to figure out which are the premium features and which are baked into the app. It's a mess. And I get it, it's not really the dev's fault, but the system is completely broken.
You should be able to see the name, description, and price of all the in-app purchases from the App Store.