Greetings from a self-published author here. Yes, you are right, books are more expensive. It is due in part to the materials needed to print them or cost. For example, with my book the cost to print it and distribute it costs me about 21 dollars per hardcover book sold. That's not including the money invested for a book cover designer, editor, formatter, obtaining ISBN, reviewers, obtaining the copyright, upkeep of writing software, and various marketing expenses. It adds up pretty quick XD However, I don't need to pay the print and distribution cost for the ebook version. Part of the reason why you find these much cheaper. If you're looking to get multiple books within a $25 kind of budget, I might suggest looking at ebooks.

Anyways my apology if it sounds like an excuse. XD Just wanted to let you know what my experience was with this.