Marvel Studios are known for believing in the motto "Look after the millions and the hundreds of millions will look after themselves". They got Robert Downey Jr dirt cheap for Iron Man 1. And negotiated the higher-paid Terrence Howard out of Iron Man 2. And the box office didn't care, they made megabucks.
So why should it be different for The Avengers? Already expected to be one of the bigger blockbusters of all time, putting The Hulk, Thor, Iron Man and Captain America fighting together and filming on the streets of Manhattan is the kind of thing that strings together all sorts of zeroes.
But certain people are saying Marvel are demanding they get it all done on the relative cheap. For something not far off what Sony paid to make their first Spider-Man film (at least in real terms).
Some people say it can't be done. Others say it can be done – even if then, in certain well heeled Los Angeles bars, they assure each other that it can't. And while some people have walked off the project, others are planning a longer game. Basically do the work, then when they run out of budget (which they believe they will) trust that Marvel Studios and their investors will pony up the extra necessary cash rather than leave the movie unfinished.
As a result of such staff changes, pre-production has been delayed weeks – possibly months. Hopefully this won't affect filming schedule and the SFX.
It seems it's Manhattan… or bust.