The credit is for a full book, regardless of book cost.
So you get one full book and then any other books you buy are discounted.
But the big reason I go with them is they give you DRM free audio files of the books to download and keep. Most books are both in m4a and mp3 format.
Considering that most audiobooks (after the discount) fall in at around $0.75/hr, you'd have to be listening to 4ish hours a day to break a hundred bucks a month (unless you listen at a higher playback speed; I don't). 2-3 hours a day is probably my personal average.
I'm not one to bash pirating - I do it for audiobooks with exclusivity deals from time to time. But I hope that if you are pirating all your audiobooks that you donate to the author directly to make up for it.