Cheap OEM versions of Microsoft Office software are available at Amazon.com. Amazon allows customers to buy various versions of the office software suite from either Microsoft or other retailers.
Use the filter and sorting options available to find a Microsoft Office version that suits your budget. The filter options are available on the left and the sorting feature is on the top right. In general, older versions are a lot cheaper than the latest releases. Similarly, there are student versions of Microsoft Office that cost around half as much. Note that it is only possible to install the student version on one computer, rather than five.