Well, here's the facts:
In 2007, http://www.zenimax.com/news_pressrelease10.25.07.htm (the parent company) secured $300 million dollars to set up ZOS and begin development by selling preferred stock to Providence Equity Partners. This was followed by an additional investment of another http://www.zenimax.com/news_pressrelease10.06.10.htm in 2010.
This places ESO's development cost at anywhere from $150 million to upwards of $450 million. As the game hasn't been released yet, most people assume the initial $300 million investment as ESO's cost to create (This is not wholly accurate, but still fairly acceptable--The studio needs offices, hardware, and there are bills to pay. Until ZOS has more than one game in development, all of their costs can be attributed directly to ESO). This is relevant because to break even from box sales, they need to sell enough copies to make back their investment.
According to http://www.forbes.com/2006/12/19/ps3-xbox360-costs-tech-cx_rr_game06_1219expensivegames.html, only about 45% of the retail "box" cost of a game goes back to developers. At the usual $60 a box, that's only $27 going back to the studio. Assuming that $300 million that was used to start the studio, ESO will have to sell over 11 million copies. By http://www.joystiq.com/2009/07/29/world-of-warcraft-retail-sales-reach-8-6-million-units/, WoW hadn't sold that many copies in the first four years it was available. Star Wars: The Old Republic (developed for http://www.gamespot.com/news/star-wars-the-old-republic-cost-200-million-to-develop-6348959) sold http://www.vgchartz.com/game/31584/star-wars-the-old-republic/ units (and was well on its way to setting records before fizzling out and going free to play), and Guild Wars 2 sold around http://en.wikipedia.org/wiki/Guild_Wars_2 copies (people assume GW2 cost around $50 million to make, but I can't find a source to back this up).