It’s $99 for a lifetime license.
The Designer’s Guide to Prototyping in Principle
Abhinav Chhikara
88224

just bought the license today (a happy customer). wanna clarify the license is up to 1.x.x as mentioned on the website.

License type

Perpetual, includes minor updates (example: 1.0.0->1.x.x updates free, 2.0 would be a separate purchase)