That's fair enough, and by comparison Cyberduck is free to download but $23.99 on the app store. Cyberduck is nagware if you download it but don't buy a license, but another option for monetization could be offering the base application for free but requiring in-app purchases for sync, ENEX import, etc. (for the App Store version). Of course there are other ways of doing it, as well, but freemium using App Store in-app purchases saves you from having to maintain your own licensing infrastructure while still offering a basic version of Joplin on the App Store for free.