L4D2 will no doubt come out for $40. Windows 7 Ultimate will be like $260. It's much easier to pay 40 bucks to a little bit of new stuff than it is to pay almost 300 bucks for a little bit of new stuff.

It's not like making it an expansion will make it any cheaper, anyways. 40 bucks is what a normal expansion pack for a 60 dollar game would retail for new. L4D2 is essentially a stand-alone expansion pack in this sense.

The only problem I see with releasing it as a stand-alone game is needing to switch games to play the original L4D maps.