How to mark an app to check the price variation It sometimes happened that some app has a discount price (christmas - black friday), it is possible to check the  price trend during the past?

Mayve the bookmarked app

It could be useful to decide if buy an app that is not strictly necessary
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).