There was a reference to an 800 million budget, but I just searched for it and Steve (as in the admin here) said he believed it was for the overall AK renovation which included not just Avatar, but also the relocated Lion King, Rivers of Light and the night package for the Safari.
I should also mention that this was early January-
http://forums.wdwmagic.com/threads/avatar-breaks-ground.878875/page-25#post-5883538
I can't find the post at the moment, but I think someone also mentioned that the River of Light budget had already been slashed (might have been
@marni1971 but not sure).
I'm not getting excited if Avatar only has a budget on par with New Fantasyland. And as Avatar has barely even begun construction (and no sign of going vertical I believe still), I wouldn't rule out the possibility for more budget cuts along the way.