I want to offer some amount of content for users to test-drive prior to purchase, but I imagine locking down more of it once the site launches (maybe 2 free practices before offering a "buy now" link, or some such.)

Currently all of the lesson content is locked down (if you've created an account using the HN signup code, then you get to see it all, such as it is).

I've gotten lots of feedback to the effect that $29 is too high for a webapp, so I certainly will revise that down before launching.

Thanks for the feedback!