Purchasing Windows XP, Vista, or 7

I'm interested in playing the game Empire:Total War on my 2009 27'' iMac. What is the least expensive method to do this? I really don't want to pay $200 for Windows 7. Do I have any options? Any help is appreciated. Thank you in advance.

Check out this site (newegg.com). Newegg.com - Operating Systems, Microsoft Windows 7, Win 7 Ultimate, Win 7 Pro, Win 7 Premium, Windows Vista, Microsoft Windows XP, Win 7 Upgrade.
You can get Windows 7 Home Premium OEM for $99.99 USD. OEM discs can only be installed on one machine, so it is always tied to your 27" iMac and can't be installed on any other Macs you may one day own.
Or Windows 7 Home Premium Full for $176.99. Can be installed on more than one of your Machines in your house.
I wouldn't even consider getting VISTA no matter how cheap it is. XP is hard to find but usually less than $100 for an OEM disc. It's a good OS, but MS will soon end support for it and from what I hear WIN7 is better anyways.

Thank you!

You can also use an upgrade version to install an OS. I just installed W7 on my Mac with my W7 Home Premium Upgrade disk, although I'm not sure if the previous user had Windows on here.