I pay $7.99 for my audible subscription per month and it gets me one free credit for an audiobook every other month. Now, if you're a penny pincher, you can use those for an audiobook that may be up to 60 or 70 hours long (like The Count of Monte Cristo) and may take weeks to finish depending on your frequency and speed of listening. However, more important, having an audible membership gets you access to their frequent sales and daily deals, where you can often get an audiobook for $5 or less.
The downside is that you end up like me and have a list of books to read. However, when you purchase them for a couple of bucks, it's really no big deal. Audible, 3-4 times a year, has 2 for 1 sales where you can get 2 books for 1 credit on certain titles. But twice a year they also do megasales where almost everything is pretty cheap. It's also nice to own your books forever vs spotify where you lose access to them. However, if you are already a spotify subscriber and don't listen a ton, I could see how their deal is worth it.