The basic version of Peak is completely free of charge. This includes a workout of four randomly assigned games a day, plus basic insights into your statistics. However note that you can only play these 4 games once a day.
You can also pgrade to Pro and get the best brain training experience available.
You can find out more about the different subscriptions available in the sidebar of the app under "Upgrade to Pro". The Pro version includes a personalised workout of six games per day that you can replay as much as you want, access to contextual workouts, as well as to all other games at any time, and advanced statistics.
The reason the app is subscription-based is because it is an ongoing service rather than a one-off app. We will keep on releasing new games as well as new statistical visualisations and comparisons of performance. We are committed to providing effective brain training for our users, and this means we are continuously working on and developing Peak. So in order to maximise the benefits to our users we must make Peak Pro a subscription service.