Why one-time rather than a subscription
Exam prep has a natural end date. You study for a few weeks or months, you sit the exam, and you are done. A subscription bills you for a relationship that both parties know is temporary, and relies on some proportion of customers forgetting to cancel.
Pricing it as a purchase means the incentive is to get you through the exam rather than to keep you subscribed. That is a better alignment for a product whose entire purpose is to become unnecessary.
What the price is next to
The $175 PSI exam fee, per attempt. The $200 to $500 75-hour course. The $170 licence fee, which doubles to $340 if you apply late. The full cost of licensure puts it in context.
Try before you pay
The app is free to download and the diagnostic is free to run. This site's tools, readiness check, math drill, and question samples. Are free permanently, with no email and no sign-up. Judge the question quality first.