If it was the book by Tri that's selling for about 60$ now and he has basically been outed as someone who vastly oversold his own skills.
But yeah there are some examples of guys I'd consider more reputable selling audio/e-books for 500$-2k (I have payed these amounts and didn't regret it). I think that's easier to pull off in poker where the link between information and the value of said information is pretty direct. Additionally those books sold at a time where many players had a base philosophy of "poker money is not real money" because after all it's just stuff to keep track of the score. Paying directly from your poker account probably doesn't sting as much as paying from the same allotment of money that you have set aside to pay other bills (speculation on my part)
The "poker coaching industry" is an interesting area of research in asymetric information though.
By the way, the most expensive ebook I've seen was a $1999 ebook by one of the best high stakes online poker players in the world. Some people complained about the price, but it sold like hotcakes. (and this was back in 2009 iirc!)