One of the first things a client will ask when discussing a project is, of course, "How much will this cost?"
There are many variables to a video project, so the initial answer is: "It depends."
I like to begin the conversation by defining the scale of the project, its goals, its intended distribution, and any known quantifiables such as number of interviews, what the locations are, whether there will be animation, length of finished piece, and so on.
It is often also helpful to know what is the client's expected ballpark expenditure. We can spare ourselves a lot of spinning of wheels if we know what the parameters are.
In the production world it's traditional to break down the budget into three phases; a brief description of each follows: