People like to highlight the effect of FX, but avatar isn't the only film that enjoy the FX advantage, all films released during 2009-2014, all enjoyed certain level of FX advantage before us dollar spike a like a gold in late-2014.
Yet we only see Avatar hitting 2.7b+ , barely any movie released during this period managed to reach half of the avatar gross although they received the same FX bump like avatar, that including the finale of harry potter series, iron man 3, frozen and first avengers. This alone can clearly tell people just how massive Avatar was given the market size.