Steam is selling Civ 4 and all the expansion packs for $10 this weekend. If you're like me and already have Civ 4, all of the expansions are 75% off.
EDIT: In fact, you can buy the $10 package even if you already own Civ 4. I purchased the whole thing, since I was going to buy Colonization for $7.50; that gave me the other two expansions for $2.50 more.