When people think of budgets they usually consider high actor salaries, location costs and fees, and special effects, all of which are generally associated with narrative films and not documentary. One of the simplest things that documentary filmmakers overlook is the general costs of transportation. Since you will not be filming on one central location, like a film set, you are going to have to move around. In an effort to get appropriate B-roll, as well as to go to many of the people you will have to interview, you are going to be doing a lot of driving. If you are going to do a nice set-up, including a full three point lighting and possibly even background graphics, you are going to have to take a few pieces of equipment with you. This may end up requiring a large vehicle or more than one. The rising prices of petroleum are not going to make this cheap, so gas is going to be a major concern. This can end up being the most expensive part of your entire film production, so try to carpool as much as possible and plan ahead. You may even have to pay to stay some places, or even rent a location to conduct interviews.