According to a report on VentureBeat, analysts at Lazard Capital Markets estimate sales to be 60 percent lower than those for World of Warcraft: Cataclysm.
"Sales of the World of Warcraft: Mists of Pandaria expansion have been disappointing, according to investment bank Lazard Capital Markets. The fourth expansion for Blizzard Entertainment's massively popular online game has sold between 600,000 and 700,000 copies at retail since its launch earlier this week, Lazard estimates."
Whether it's World of Warcraft fatigue, the nature of the expansion, or perhaps Guild Wars 2 that's to blame is up for debate. No official numbers have been released by Blizzard.