That's right, completely free! The drawback is that only books available in the public domain are available, but there are a lot of them. Many more than you might think with many of them being very good books, too. You can download in almost any format and easily burn an audio CD or put them on your portable music player. Check out LibriVox if you've been looking for this kind of thing for those long car trips.
I'm told some readers aren't so good and some are very good, so if you find a reader you really like you can go back and search for other books done by that particular reader, too. I believe you can also choose to omit bad readers from your searches. And if you like the service and use it, try to give back by signing up to do some reading. It's pretty easy.