So 1 credit is worth 1 book (any book available on Audible no matter what the price retail)?
1 credit will get you any book - I use the credit for the higher priced books and purchase the lower cost ones outright.
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.