My previous comment may have insinuated towards an act of piracy for which has led to a misunderstanding with the moderators, however let me a bit more specific this time:
I had assumed that you were familiar with offers made by some audio editing software providers, essentially offering a free copy of windows 8 as a sweetener for the purchase of their products and a 12-month subscription to their product updates. However my statement may be out of date because last I glanced upon such an offer was autumn last year, and that was only for professional grade software.
However if that is not possible with you, I had heard a rumor that you can get a serial number (only a serial) for as low as 39.99$ from some sources completely legally. But this is only a product key mind you, ISO of the OS itself is available for download on the Microsoft product page for free!
There are also students "giving" away serial keys in exchange for helping them... "expedite" their diplomas or even assisting with surveys for their diplomas, some students are also selling off their books and along with a windows key or two.
There are many options to come off cheaply with a legal windows copies, you just have to find your way.