How can I purchase an affordable copy of Office?

[ntco]

I am the IT person for a medium-sized warehouse. MS Office is used extensively around here and we've been recently having obscure compatibility problems between office XP and 2003. I would just like to bring everyone up to 2003 (there are only 3 people that are still at 2000/XP). However, I am having a tremendously hard time finding a reasonably-priced copy of Microsoft Office, and the prices seem to fluctuate wildly between different sources.

I've tried: amazon.com,buy.com,newegg.com,cdw.com,pricewatch.com,etc...

I'm seeing prices anywhere from $200-$500 which in my mind is ridiculous, especially for an older version. I have seen random ads on the internet for cheaper options, but I wonder how legal they are. If I'm just going to buy an illegal copy anyway, I might as well pirate my own copy for free.

Buying a separate OEM Preinstallation Kit (OPK) and Media License Kit (MLK) seems to be a fairly affordable option, but since it's geared toward newly-built machines, I'm wondering if it'd be illegal to just install it on an existing machine. And what's the difference between this and a regular OEM copy?

Does Microsoft really have to make it so convoluted?

Any help or advice one could offer would be greatly appreciated!

Thank you!