I've read somewhere that a low budget film is like 250 to 500 thousand dollars and a no budget is around 20 to 50 thousand dollars...but that's way back when I read this article.
If you have a talent for talking people into working for free and lending you their equipment and expertise... securing locations for free, getting props and makeup donations, writing a super clever script around the fact that you have no cash, etc....
... then it is possible for it to be $0.
If you figure out how to do this, please PM me the secret (I'm not kidding).