I created an account on my iPhone, but it doesn’t show me the price.
The use of the App is free (it is part of the Acronis ATI 'package'), all you need to do is to log into your Acronis account in the App. As far as I know it works for both subscriptions and one off purchase.
You can also use the mobile app even if you don't have a desktop version. Just download the app from the store, install it and proceed as per https://www.acronis.com/en-us/support/documentation/AcronisMobile/index.html#40296.html