Here comes to the factors which contribute to the high textbooks cost.
Many information tell that producing a textbooks is not simple and relatively expensive.
There are some reasons breaking down:
The author’s work got copyright and which restricts other parties from publishing the textbook and sale at the same market. A poetry anthology for a literature class, for example, may involve clearing hundreds of copyrights.
Highly specialized material
Many college textbooks are highly specialized and the material is unavailable in any other book. The low volume of published books and the lack of market competition drive publishers to jack up prices.
The book stores does make profit by selling textbooks.
The publisher always updated a new edition by adding its content, change the numbering and leading the previous edition hard, or impossible to sell.
The additional materials like tutorial CDs and workbooks contributed to the rise of the textbooks’ costs. Publishers say they have increased investments in the development of supplements, particularly web-based tutorials and self-assessment tools, to meet the increased demands from instructors who hope to enhance student learning (GAO 2005)
Lastly, here is a video from youtube, as a supplement, telling the reason why