I don't understand why producers think a writer is even minutely qualified to do a shooting budget. My thinking is that the producer should ask his production manager what it'll cost to make. The truth is ... there's a range. The producer can do it ... or do it up right. That said, here are the things that really jack up the cost:
• Big cast
• More than 3-5 locations
• Special effects
• CGI work
There are books and courses on how to budget films and television. If you're really interested, get a book and read up on it ...
http://www.talkfilm.biz/onlinefilmcourses.htm ... eBooks and online courses
http://www.amazon.com/Film-Scheduling-Budgeting-Workbook-Yourself/dp/0943728142 ... by Ralph Singleton, course $145 new, $22 used
http://search.barnesandnoble.com/Film-Budgeting/Ralph-S-Singleton/e/9780943728650 ... by Ralph Singleton, budgeting only
Google is your friend.