Just went to cancel my Audible membership after the £15 free Amazon credit deal and after choosing the "it's too expensive" reason you can pause your membership or choose one of the following offers:
1. 50% off membership for the next 3 months. (making it £3.99)
2. A free credit for a free read.
I assume this will appear for everyone who selects the above cancellation reason which seems like a good way for existing users to get a nice discount or a free book. :)