The casting process on a low-budget independent film often gets short shrift. Don't make that mistake!
Considering how important casting is to the overall success of a film, it's surprising how often first-time producers and directors fail to budget adequately for it. Obviously you need to budget a casting director (or assume that responsibility if you don't), but there are a few other things that you need to consider:
Paperwork. Copies of the script, sides, headshots and resumes (even when I receive resumes online I still print them out for auditions), audition schedules... put in some money for reams of paper and toner.
Advertising. A lot of online boards are free (Mandy.com, ActorsAccess.com, Breakdown services), but others charge. If you don't happen to live in a city with a large pool of local actors, you may have to take out a print classified or put up ads at theaters.
Assistants. Whether you pay them or not, consider having one. You can't do everything at the casting session. The assistant can wrangle the cast in the waiting room, set up the camera, and/or help manage the bookings and paperwork. If you're not paying this person, budget some food/travel money.
Casting Space. Don't assume that you can get this for free. And if you're a SAG film, you expressly can't use your apartment or house. You wouldn't want to anyway. You need at least two rooms: one waiting area and one audition room, with some kind of wall between the two.
Some casting directors have their own casting spaces. Others don't, so you'll have to rent from a local performance space. When I've had to cast in areas without a place, I've rented from conference centers, community centers, local theaters, and convention halls.
You'll usually see 20-40 people per lead role. You can see up to 6 people/hour during the first round, and 3-4 pair/hour during callbacks, so budget your rentals accordingly. Schedule lunch and stretch breaks!
Audition Recording Storage. Make sure to budget for a backup drive as well as recording media. You'll want to back up auditions so you can watch them later and (possibly) archive them.
Rehearsals. Yes, you have to pay for rehearsals with your actors (assuming you're a SAG film). But it's not just the salaries - you should budget for working meals and snacks. You may want to have a full-cast reading and invite the crew. I recommend this, as it's (a) the last time anyone will get a chance to "see" the entire piece before production, and (b) you'll probably hear script issues that need addressing.
Meetings. It's also important to take time out to meet the cast, take them to coffee/lunch, and go over the role with them one-on-one. Budget for some food/coffee here.
Legal. By far the most amount of time your production counsel will spend on your film will be on the cast deal memos. It is money well-spent. Credit placement, profit participation, additional credits (such as co-producer or associate producer), nudity clauses, etc. - these contracts can get complicated fast. This is a good reason to have a production counsel in the budget (at least for a feature).