World of Warcraft's Mists of Pandaria will go live on September 25th for $39.99 US from major retailers and from the main website for the same price. You can upgrade to the Standard Edition and Digital Deluxe Edition ($59.99 US) through your Battle.net account page now. There will also be a Collector's Edition for $79.99 US.
Look for more info to come as it develops.
[Source: Blizzard press release]