Generally if a book is around 8 to 10 hours of audio the price for the book reaches the price of a credit. All books can be bought for a credit or cash. I of course choose cash for short stories. I just finished a 27 hour book for a credit.
All books cost one credit, there are no 2 credit ones.
With a little searching you can obtain many audio books for free. And of course they are also available to steal on bit torrent. But I personally want to pay the author and reader for their work, and frankly the Audible app works so well I dislike the klugey methods to off brand books.
I too use Audible. I like it, I listen to it at bed time I have to remember to set the timer otherwise I might drift off and have to back track, which is a pain.