This is a subscription based product. Unfortunately or by design the website or the description in the Mac App Store never mentions what the price is. However from the list of popular in-app purchases I would offer an educated guess of $24 to $36 a year depending on the subscription length of a month or a year. This of course means that if you use the App for 10 years, which is not an unrealistic time for using it, it will cost you a minimum of $240.00 to have continual access your data assuming the price is never increased and the company stays in business that long. However should the company fail, then all of your own data might no longer be accessible. You might be able to export it, but to what?
This is an example of why I detest subscription marketing models that store user data. Once used you are committed to paying for that App for as long as you want to continue to access the your own data that you provided to the App. Another way of looking at it is that your are held hostage to paying for the App so long as you wish to access the fruits of your own effort and the company stays in business. This is why I never support subscription based Apps unless they are simply providing a service where user data is not being managed by the App.