We are also going to publish an audio book, but the audio POD industry really doesn't exist, so we are doing it ourselves. During the first recording session, we learned typos and other written goofs jump off the page. (sort of a backwards way to edit, but it sure works)
There is a $200 charge for that. We hadn't considered there might be.
No complaints on my part; just inexperience raising its head. ISBN's come in blocks of ten and go for $300 - so there's $30 right there.