If tomorrow we had hyper-inflation and a bus ticket cost 1 million dollars, would anyone still be impressed by a billion dollars in profit? Would the mediocre comedies netting trillions at the box office be given the title of "most profitable movie" of all time?
A dollar in 1977 had the buying power of somewhere between 3 to 4 dollars now. $1/3 billion profit in 1977 dollars is a significantly smaller number.
The rule of thumb is that over the course of a typical film, the distributor will receive about 55% of box office gross. Foreign deals can differ a fair amount.
Each deal is different, and it's hard to say what any one film gets without some insider info.
Then there's the cost of the film. There's not been a firm number on Avatar's production costs. High 200s is a good guess, but it's a guess.
P&A (prints and advertising) can be a huge cost on something like Avatar, though it had fewer prints than a typical blockbuster. Advertising was certainly substantial, though. I suspect 100 million is actually low.
Cameron's share is likely defined to come after some sort of breakeven point, some minimum number the studio beancounters said had to come back to the studio. There's really no standard whatsoever for someone like Cameron though, so who knows.
It's also important to remember that a lot of a film's production cost is really just the studio paying itself. Soundstage rental. Grip trucks, costuming, blah blah blah. Money moving from one column to another.
Avatar probably didn't qualify for any tax credits, though some films get really amazing deals. Some come close to having half their budgets in transferable tax credits, depending on the state or locale they shoot in. Those tax credits can be sold for cash and can considerably lower the cost to a studio.
The traditional hope for an average film has been to break even on box office, profit on the ancillaries - dvd, tv, etc. But with DVD sales dropping, that model will need some adjustment.
Avatar, though, is probably pretty close to some definition of profit in theatrical release.
And all that billion plus dollars is cash flow. Which is king, as much in Hollywood as it is anywhere else.