It might all depend on how much CGI they plan on using because CGI can really ratchet up a budget.
Personally, I don't believe the film should really require all that much CGI to get the story across. But then, I'm not 12-25 years old...
As for things like Burton's and Depp's salaries, I have no idea what their practices tend to be, but some directors and stars will completely forgo a salary in lieu of a percentage of the gross, particularly when/if they're working on projects that are close to their hearts, which this film would certainly seem to be in at least Depp's case.