You're as valuable as you make yourself to be. If you want an absolute bottom, calculate the gas cost and everything that is tangible. After that you have to create a per hour rate, it's important not to get carried away thinking you need to charge a lot upfront, as i'm sure you'll realize this won't be the only gig from the buyer/client. But I would think somewhere around $150-200 is a good start for one film, always ask for referrals, and if you think you can get away with charging more then do so but only after you gain a client base and in increments.