What Welsh Gambit said is basically right.
You'll want to do the duties of a Line Producer / Unit Production Manager. To get a budget, they basically break down the script, scene by scene, working out all the details of what is needed within each scene, from that work out some resemblance of a schedule, which leads you to how many days each person is working, and then working out what everything will cost. Craft services, equipment rental, location fee's, insurance, permits, above the line costs (writer, director, producer, leading cast), below the line wages, post production costs and more. Not to also mention development costs that are already sunk.
It won't exactly be accurate as there are a lot of variables that won't always be known until later on.
Though I suspect you're talking about something a lot smaller. How much smaller depends on what you have in mind. If so, please provide more details.