The first expansion for Civ V - Gods and Kings - is being released on Steam tomorrow in the US (well the 20th), and will be available to us in the antipodes on the 22nd (so the 23rd).
The expansion ads in a couple of of things that have been missing from this version of Civ, mainly religion and espionage. It also adds in some new playable civs and includes some back-end enhancements that apparently allow you to play a full game beyond the Renaissance without your system attempting to replicate a x386. There is also an alterative history mode called Empire of the Smokey Skies that is very steampunkish.
No idea on the cost but I understand it will be around $35.
There are a couple of good reviews up already so you can read them at your leisure:
Wot I Think: Civ V ? Gods And Kings | Rock, Paper, Shotgun
Civilization V: Gods & Kings hands-on preview MyGaming News and Reviews