The app is free. In fact you don't miss much by not paying the subscription.
Pro features are $10/month, but are actually also "free" via an unlimited time trial.
The $10/month price tag is to support the developer if you're so inclined, which includes some nice-to-have features.
The author did say he experimented up and down with prices and what people would pay for. If people think they are getting more then 33 cents a day worth of productivity out of the app ... then it works for them. Does seem rather high to me as well.