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It's increasingly common. This particular one is a bit of a headscratcher since typically pay-once-for-an-update period is presented as a de facto subscription, but this also has a subscription.

In this case, you'd pay $150 for an 18-month window of updates, or $144 for two years of updates (or, if you like, $72*1.5 for the same 18-month window, assuming you're going to pay another $150 at month 19 for something interesting.)

Contrast that with Agenda, which is $35 to buy with 12 month of updates, or about $100 for a life-time of updates. The tradeoff is more straightforward as you're just deciding whether to bet on more than three years of features.

Or contrast with OmniFocus which is $150 for the major version, which typically is on a 4-5 year cicle, or $5/mo. In that case you're just betting that you'll use the current major version more than 2.5 years.

(I'm ignoring cashflow discount; you get the idea.)

(The app itself is fun and fast to use, and I'm not complaining or demanding special treatment. I'm just interested in how these things are priced.)