I totally agree with you that we're outside the impulse-buy price range and we'd sell more copies if we charged less money.
However, our goal from the start was not to make a tutorial for people interested in backbone. Our goal was to make a more in-depth collection of great patterns for intermediate to advanced backbone developers.
Hackers are smart, they can read the docs and figure out the basics. We wanted to share our experiences with working with backbone for an extended period of time.
Our book is a great resource for people who would use backbone professionally and at $24, that's cheap.
Check out the excerpts, and read the reviews, play with backbone, learn it. When you're ready to bet your app on it, pick up the book :-)
xoxo ngauthier