Why do you think it's so cheap compared to other Windows versions?
Lots of licensed technology related to multimedia files has been removed from the base of the OS, reducing the price.
The price has been reduced further as a result of a continuation of the trend specific to the recent releases of Windows. Vista was introduced at a very high point (mostly for being a huge generational change), 7 at a slightly lower point, and 8 at an even lower price point.
In addition, the price has also been reduced as a result of the introduction of Windows Store, through which Microsoft can gather income after the sale of the OS happens.
In the end, the customer / user wins. :)