Given the difficulty of finding and earlier version, and the new features in CS6 that might be of use to you if you decide to move into publishing to mobile devices (and improved epub export), you'd have to be getting the earlier version at a real discount to make it worthwhile.
Creative cloud is a subscription service. For about $49 month (if you subscribe by the year) you get access to all of the programs in the Creative suite, and some additional stuff, and you get any patches/upgrades as they become available. This is far less expensive than buying an upgrade every year to one of the Suite packages on a conventional license, but more than upgrading a single product. It's definitely worth considering, and may become the only option in a year or two if the market accepts the subscription model.
