It's a one-time $25 cost. It is also entirely worth it. iOS users spend an order of magnitude more money than Android users, so that was the creator's logic -

It's a one-time $25 cost. It is also entirely worth it. iOS users spend an order of magnitude more money than Android users, so that was the creator's logic - use them as the "whales". It's 100% fine and honestly he should be charging a subscription for such a great app, on every platform.