Windows 8.1 Update is now available to OEM manufacturers at $15 instead of $50, a $35 or a 70% discount on the software costs. For phones and tablets smaller than 9-inches diagonally, OEMs will be able to put Windows 8.1/Windows RT/Windows Phone 8 on there for free.  All that OEMs have to do is stick to a minimum resolution of 1280 x 800, 1GB of DDR3 memory, a dual-core processor and 16GB of storage space for tablets, as well as leave the default options for search on Microsoft’s Bing search engine and keep the default browser as Internet Explorer.

Source: Windows blog