OnLive officially launched its cloud gaming service in the US and released a list with details on how much it will cost to rent games. Pricing and available rental durations vary for each game, some games like Batman: Arkham Asylum can only be rented for 3 or 5 days for $4.99 and $6.99, respectively, while games like Assassin's Creed II offer an "unlimited" pass for $39.99.
The initial range of pricing options includes rentals that last three or five days and an unlimited pass that provides access until at least 17th June 2013 - presumably the length of the current licensing agreements.
Buying a game through OnLive entitles you to access that game for the length of the agreed licence, providing you keep up your OnLive subscription. If you allow that to lapse, you can't access the games again until you rejoin.
You can check out the full price list at Eurogamer. Besides the rental cost for games you'll also have to pay for an OnLive subscription, people who subscribe now will get a free one-year subscription, but it's still unknown how much the service will cost afterward. Currently, OnLive is only available in the US but the company has plans to roll out its cloud gaming service to Europe by late 2011.