If your book does what it says, it's going to mitigate some of the risk of deciding what path to go down when starting a startup. That's potentially weeks/months/years that you might be saving me from chasing after an idea.

That's worth a lot more than $15 (and especially $3.75)

I'm selling my new book for $39 - and that's a discounted, prepurchase rate! But the value proposition is "if you read this book, and do what it says, and happen to raise your rates by even a $1 - that's another $2k in your pocket this year." Emphasize that your book will help people not waste time chasing the white rabbit, and you can charge a premium.