That’s a ‘how long is a piece of string’ question, I’m afraid 
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If you have two talking heads in your kitchen with no special lighting and a camera you own or borrowed and you can edit it yourself, then nothing but time.
But if you have a sci-fi special effects laden epic, then millions!
The only way to know how much it will cost you is from looking at the script and breaking it down. Does it have many actors? Many locations? Many props? Any night time shoots? Are the locations available to you? Will you need makeup? Special effects? etc.
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