Much would depend on the material you already have for them to get started with, and how much work is involved. A simple action story is a lot less work then a historical drama.
Ghost writing means no credits, which can be good as gold, so we like to make up the risk in cash.
You might pay anywhere from a grand to 15 grand, maybe more depending on your position to get the film produced, and how much money you intend to raise. If you come up with a million and pay 1.2%, then you're looking at 12,000 bucks. You may negotiate, of course and maybe get away with a grand. Ish.
Heck, could even be little as 500.00 depending on who signs on.
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