Apple App Store currently has these two titles on sale for $5.49 each. Note that this is for Apple only. They are currently $19.99 US each on Steam (although Steam versions can be played on Windows and OS X)
Civ IV: http://itunes.apple.com/au/app/civilization-iv/id435169692?m…
Civ IV, Colonization: http://itunes.apple.com/au/app/sid-meiers-civilization-iv/id…
I know these are older titles, but thought some people may be interested. I am NOT sure when the sale finishes, but have put expiry as end of February as I think Aspyr sometimes has "end of month" sales.
Don't forget, iTunes/App Store credit can regularly be bought for ~25% off, so this would make these games even cheaper.
EDIT: still on sale so extended the expiry another week
Civ IV yes, Civ IV: Colonisation, no.