Actually, they are free. I am a graduate student in Mathematics and there is no book, on any level of mathematics that I have been unable to find for free download.
Places like www.avaxhome.ws and old.pdfchm.net are chock-full of math textbooks not to mention the torrents which can be found on isohunt.com and thepiratebay.org that consist of collections of thousands upon thousands of graduate level mainstream and esoteric mathematics texts, all high quality scans.
In total, I have many thousands of dollars worth of mathematics books, all of which I have obtained for free (and I feel justified; there's simply no cost-effective way to obtain those books on my side of the planet).
