I've worked for a publishing company before, and the whole process is tedious and meticulous. It all starts with a plain manuscript, which will be proofread by professional editors more than twice (goes back and forth to and from the author) until everything is perfect. For once, this is very expensive, not to mention that the rates may vary depending on the total number of words.
It then goes to the layout, and it'll take longer if the book has a lot of images, making sure that all pages do not have cut words, or images do not affect the entire layout and text of the book.
Now, to the more expensive part, is the printing. For the book cover, it is more expensive if it is going to be print in hard cover as opposed to soft cover, which would also depend on the materials that will be used, as well as the paper quality.
There are many other steps in between those I've mentioned above (book cover design, interviews, marketing before the book gets out, marketing materials, sponsorship engagements, etc). Books may appear simpler when we read them, but there is a lot more things that are being done behind the scenes :) Oh, I forgot to mention that every single time an author wishes to reprint their book, they have to pay their publisher too for royalties, since you can't just directly go to the printer just to have your book reprinted.
Here are some links to how the publishing process works, justifying why the larger the books are, the more expensive they are:
A nice image that details most of the steps: