There are two established ways to buy Microsoft Office on the cheap. One is the OEM version. Aimed at PC makers and do-it-yourselfers, this edition is entirely legal for consumers to buy. With it, you can get the Home and Student Edition for $80, or the Standard edition for $225, which represents significant discounting.
The disadvantage is that you can only install Office on one PC. Full retail versions let you install the software on two PCs, except for Home and Student versions, which allows 3. And if you have to replace a defective motherboard, it's likely Microsoft will count that as a new PC and disable the software.