Psyringe: Civilization IV Complete is 75% off at GamersGate - 6.24€. Highly recommended for strategy lovers who don't already have it! :)
If you happen to live in a place where Amazon is an option, it's worth noting that
their version is DRM-free. Alternatively, they also offer a
Steam version, so be careful that you make the right choice.
It was actually on sale a while ago (technically, it still is, if you call 25% off the DRM-free version a sale), but it isn't 75% off right now, I'm afraid. And again, it's irrelevant if you have regional issues with Amazon. I thought some people might like to know this before they take the plunge. Civilization IV is an excellent game, and the complete edition is really the only way to go. The expansions add a great deal to the game (one is a remake of Colonization!), and also enable several mods like Fall from Heaven II.
For the record, another user, I cannot recall, had argued with me in this very topic that GamersGate does not lower the price of a deal during an ongoing promotion. Civilization IV has been 50% off all week, and now it's 75% off in a daily deal. So yes, for the record, they are not different from Steam in that regard. It often pays to wait until the last day... unless a game sells out or the discount is a mistake, mind you. ;)