In stores in the western world, you'll be confronted with three Windows versions: Home Premium, Professional, and Ultimate (although that last version might be hard to come by). NewEgg has published the pricing information for all OEM variants of those three versions. Putting all the three price variants together for the three versions (get it?), results in the following table (as far as I can tell, these prices go for both the 32bit and 64bit versions):
These OEM versions have historically always been cheaper than the other versions. They are intended for small system builders, but they are available to us mere geeks too. They do have a few limitations, though. You get no support whatsoever, and you can only perform a clean installation.
In addition, the license limits the OEM copy to only one machine. In other words, install it once, and you're stuck to that machine. Then again, this is a limitation imposed via the license "agreement", and as such, isn't worth a whole lot. The same applies to Microsoft as it applies to Apple and Autodesk: software is sold, not licensed, and as such, you are fee to transfer it to another machine. Usually (but I could be mistaken) calling the phone activation system will do the trick.
In any case, these OEM copies are a relatively cheap way to get your latest Windows copy, and they are usually quite popular. Note, though, that actual, big boy OEMs (Dell, HP, etc.) are paying a lot less than this per Windows 7 license for their retail machines.