I just found this book announcement for Introduction to Network Simulator NS2. Even with ns3 on the way this is good news. However a book that is written for undergraduate and graduate students must have a cheaper paperback edition costing less than $100.
Introduction to Network Simulator NS2
Published
Managing organized complexity 
$100 is the average price for technical textbooks.
I recently got the PRML book for ~$80 and the Algorithmic Game Theory book for a similar price.
In several cases the price is even higher—especially undergrad textbooks that you would only use once (e.g. Physics).
The new textbook price is part of the reason that there is a substantial second-hand textbook market here, especially for non-reference books that are useless after the class is over.
@thanos:
At least Nisan’s book on Algorithmic Game Theory is available online [pdf]
I know, I have that version too. But I wanted the hard copy as well. I can afford it ;).
Re: non-printable PDFs:
http://kotsokalis.wordpress.com/2009/05/23/printing-non-printable-pdfs/
…although, for books, I also like buying them. It’s a proper “thank you” to the author.