As with any commercial business, we periodically do market testing with prices that vary from our norm. This is the only way that we can test prices on the App Store. It seems you have encountered one of these tests.
If you're already a subscriber and want to see what options are available for you, go to the Settings app > Apple ID > Subscriptions and tap the name of the app. You will be able to see what subscription options are available to you.
If you do not have an active subscription and are interested in seeing what is available to you, we recommend starting a free trial for the app and then repeating the steps above.