So if I have a screenplay for a 100 million dollar budget I shouldn't bother write it just incase I get a shot?
No.
You should definitely write it (unless it's the Catwoman script) because directors and producers are often looking for the thought process or approach a writer has regarding a story's construct.
Your spec script opus is very unlikely to be produced as is.
It would be rewritten to accomadate budget, talent, locations, and director/producer visions of the story.
Even with the "locked" studio version the finance guys agree to the cast will have five different ways to deliver the lines, actual secured locations will force some changes, the editor will franken-film the shots, the distributor will want changes, director's cut vs. theatrical release cut, then there's the MPAA changes and what pre-release focus groups will change.
Ever watch those DVD extras of out-takes and deleted scenes?
Those were all scripted, paid for to be acted and shot, then edited, then deleted for "pacing" or whatever.
That could be hundreds of thousands of dollars per screen minute just wasted on the editing room floor.
So... no.
No one's screenplay is some holy writ that cannot be blemished.
Go ahead and write your story.
Show directors and producers how you think.
Often what happens is that you may be asked to rewrite someone else's near-hit, almost there story.
That's where script doctoring comes into play.
Actually I have an idea for a Catwoman script, but I'm not sure if I should write it due to getting the copyright and the high budget.
Don't even waste your time.
Outline it and shelve it.
Move on to something original and uncomplicated by rights and legal mumbo-jumbo.