You can get book credits with a monthly subscription, I think when I was doing that it was about as good a deal as audible. libro.fm also has the advantage of being DRM-free from the start
Oh, right, I do the subscription, so it's $15/mo for 1 audiobook, plus 30% off further purchases. I guess I don't "audio-read" that much, so that's a good deal for me. (I actually have credits saved up, which don't expire.)