Place an ad in newspapers etc, giving "the chance of a lifetime" for people who want to be zombies, you could charge them $1 each. I would guess that there are a lot off zombie film fans who would love to be in a movie (50,000?), even if they don't get paid.
Not only would you get $50 thousand, but you would have an army of zombies to shoot some awsome scenes.
Being a zombie would be hard work - but you don't tell people that, and some zombies would quit, but you keep their money.
If you are less optimistic about zombie fan turnout, you could charge up to $5. I would pay $5 to be a zombie.
Later, you could write a book about how your zombie film was financed by zombies.
