How to mark an app to check the price variation
You can check the app on AppBrain, which provides an alternative frontend to Google Play. Their app pages a.o. feature a history section. On the example of aCalendar+:


App history (source: AppBrain; click image for larger variant)

As the screenshot shows, you get a price history along with the changelog (right part) if the app was paid – and you also get a rating history (left part).