someone online was selling them discouted.. possibly rpg.now . the sale might be over.
also, nobleknightgames always sells things with at least a 10% discount. they sell new and used items, with used items discounted more.
amazon.com sells the rpg books, with competing sellers with different prices.
the books are over-sized, the pages are heavier than most, and the over-all work is higher quality.
also for the truelly cheap out there, the books, all of the books, rpg's and audio books are on torrent sites. [if that's your thing you probally already know that].