Commanders and Comrades! Steam are again offering, for a very limited time, the Tiberium Wars, Kane's Wrath and Red Alert 3 titles at the knock down price of 2.49 UK Pounds each. The offer ends today, so you'll really need to hurry. The offers are linked below although you may find that the prices vary from region to region.
The titles ought to be compatible with
C&C:Online, so if you're looking for an affordable way to get online in those games, this is probably it. You will need to have the Steam version of Tiberium Wars installed if you wish to play Kane's Wrath on Steam.