I released a mobile app for checking upcoming train timetable half year ago and earned approx $300. Assuming the same sales figure on the next half, yearly will

I released a mobile app for checking upcoming train timetable half year ago and earned approx $300. Assuming the same sales figure on the next half, yearly will be $600. Was inspired by a previous "passive income" posts in HN too, original goal was to cover Apple developer annual fees.