Age of Empires Online is out and available to play for free, offered so far are the Greek and Egyptian civilizations with limited access for non paying customers.
Both Premium civilizations will cost you $40 and $20 (£14.99) seperately with other minor packs at around the $5 mark although at this time they seem absent from the Games for Windows store, but microsoft are also offering the Season Pass for $99 or £79.99. The Season Pass will give you six months of access to all the premium content, although whether the pack will stay at that high price is yet to be seen while the game is still new whether they can afford to keep the price so high.
For those playing for free will find things like achievements requiring premium content to be able to earn them and only lower levers of loot can be equipped for free and the amount you are able to hold. It is still the very early stages of this MMO and things could change depending on player numbers and how much they are willing to spend over time.