Pricing for books can't be undervalued across a broad scale like that. There is no telling how much people will pay for a particular book about a particular topic. Obviously cost of physical production plays a part, but also the amount of work that goes into its content creation. Then you have print run, quality, amount of demand, etc that all factor into its "value."
A certain book to the right people can be worth far more to them than you might ever consider paying.
For example , I paid $550 for the book 'Roubo on Furniture.' Years of research went into its creation. It's of a physical standard that is unheard of in today's world. Only 1000 were made. It's of a topic that is of personal interest to me. It makes a hell of a addition to my personal library. All of these factors justified the price to me personally, where as I'm sure the next person might balk at it.
As for your D&D book example, they cost that much because enough people are willing to pay that much for them. Therefore, that is what they are worth at the very least.
Will enough people buy this book at this price? Hard for me to predict, but it's a niche topic for a select group of interested people.
For me personally, in a world where content is increasingly digital in nature, it's nice to have something physical in hand like a book. Considering the very nature of a keyboard's tactile experiences, this book seems rather fitting too.