Save $10. Use the following Applescript that does exactly the same thing (less choosing the voice). I forget the original source - I apologize to whomever first came up with this.

say (text returned of (display dialog ¬
  "Enter what you would like synthesized into a file." default answer ¬
  "Welcome to Mac OS X.")) saving to (choose file name with prompt ¬
  "Choose where you would like to to save the file." default name "say.aiff")

By the way, if you want a sample of what a somewhat large amount of text sounds like when read through Apple's speech, check out http://www.drivingmenuts.net

It's a work in progress.

Have you *tried* it yet though? Listening to a whole book with any one of the apple voices is like vigorously rubbing your head on a cheese grater. Apple's speech synthesis, at one time the best on any consumer machine, has gotten dusty and is way behind most of the commercially used speech synthesis. Listening to anything more than a few sentences of text will drive you into a killing frenzy.