I've suspect that some movies get ripped off and others don't.
Forensic accounting isn't my field. But it does seem that in some fashion or other money in the budget doesn't actually move on to other hands. I suppose simple kickbacks are not heard of but by all reports lawyers and accountants are paid huge amounts of money to claim that movies lose money. Inflating the budget seems to be a way to do this. It appears instead that in fact most movies make money eventually. But taking a year or two means not making a lot of money.
Also, from CaptJimboJones
Keep in mind, also, that blockbuster-type movie budgets often include years worth of development, including the cost of options for scripts, directors, actors, etc., that may have no bearing on the movie that was ultimate produced. I believe this was a large part of the price tag of Superman Returns.
This too is true.
I would add that marketing costs can be quite considerable. Wide releases can cost quite a bit of money just making that many prints! Merchandising income for some releases is quite high, though. However these figures are pretty closely guarded it seems.
In short it seems that reported budget is not even a very accurate guide to the physical quality of a film (sets, costumes, FX) even though that is something that can usually be bought.