Good news! Mona will be a one-purchase app and will be available on App Store this Spring:
Free trial: 14-day full-feature access for free, with no recurring fee.
Mona Essential: full-feature access on a single platform (iPhone, iPad, or Mac) for 10 USD.
Mona Extra: full-feature access on 3 platforms (iPhone, iPad, and Mac), and supports family sharing. Mona users can upgrade from Essential to Extra for 7 USD.
If you have purchased Spring or Spring mini, Mona Extra will be just 9 USD.