The thing that some seem to have missed about iBooks Author is that it is an assembly tool, not a creation tool. It's designed to make it easy to pull together the parts of a digital textbook into whole. It lowers the skill level required, and that is a good thing.
But it doesn't actually help you make the parts. You can embed video and audio, but first you have to produce them. You can add a 3D model, but first someone has to program it. And of course you can have text and pictures, but first they have to be written and edited.
The only way I could convince myself that there were savings was if I ignored the significant cost of producing the components. For example, take the sample textbooks that you can currently download for free from iBooks. I've downloaded none of them due to the size; they're all 1GB or more. But that size tells me that they're all chock full of video, models, and other snazzy content which was not cheap to produce. iBooks Author could have saved how much, exactly? Whatever it is, I bet the savings are actually less than any single production cost.
BTW, I know the costs of particular textbook project. Back in December I posted about a trio of high school teachers who wrote their own textbook. They took parts of an existing CC licensed stats textbooks and wrote the parts they needed. They had to change it so it met MN state standards, covered the necessary core requirements, and used examples that their students would understand. Their total cost to produce the textbook was round $25,000.Most of that went to pay the teachers' salaries for the time they spent writing. Would iBooks Author really have reduced the cost any?
Now, I will grant you that iBooks Author will help in the final stages of making a digital textbook, but I do not see how it can help the overall production cost. You cannot even use it to organize the project, much less make any of the parts.
Would someone care to prove me wrong?