No, not every book on Audible is 1 credit. Audible offers a range of subscription plans for listeners that allow them to purchase and access audiobooks. Most Audible plans offer a specific number of credits that can be used to purchase any book in the Audible catalogue. Each book will cost a different amount of credits, depending on the price of the book. Some books may cost more than one credit, while others may be discounted and cost less than one credit.
The best way to find out the cost of a particular book on Audible is to add the book to your cart and view the price before you buy. You can also view the credits needed for each book on the book's detail page, as well as on the Audible mobile app.
If you're looking for a cheaper way to listen to books on Audible, you can check out the Audible Escape and Audible Plus plans, which offer an unlimited number of books for one flat rate. You can learn more about these plans here: https://www.audible.com/plans.
Hope this helps!