Filmmakers are often surprised by the costs associated with vfx work, especially when it's an expense that wasn't planned for. The best way to keep these costs down at this early stage is to spend the time, and sometimes some money, to understand and plan for your vfx needs during pre-production.
Treat visual effects like any other line item for your production - breakdown your script, analyze your projected vfx needs and budget appropriately. If you need help with cost forecasting or understanding your likely needs, try to enlist help early. This will give you a realistic number to work with, not an obligatory line item in your budget.
• For projects that are not vfx-centric, the general rule of thumb is that 10-15% of the total budget should be earmarked for fx. These are projects where the visual effects are a compliment rather than a focus: composites, clean up, simple CG enhancements, minor set extension, etc.
• For more vfx heavy projects, generally figure 20-25% of the total budget (up to 50% on tent-pole projects). These are projects where vfx plays a central role: CG Characters, significant CG environments, heavy fx sequences (think avalanches and tidal waves), etc.
Involve your vfx team now.
The way to save money is to attack the issue early. Even projects with limited vfx needs can generally benefit from getting input from your vfx team as early as possible. Is it better to shoot that green screen outside or on stage? Should you find a different location or can the one you have be extended to look the way you want and stay within your budget? Find someone who can help you figure out the best way given your shooting and budget constraints. Designing your vfx shots in pre-production will help prevent unexpected costs later.
If the project plans to hire a production-side supervisor and producer, bring them on now to help break down your script. Even if they don't start full time, have them consult for a few days and start to work out your needs and discuss shot design; it will be well worth the few thousand dollars you're likely to spend.
If you're not planning on hiring a production-side vfx supervisor and producer, reaching out to possible vfx vendors is another tact. Most vendors will help you breakdown the script and provide you with a bid, and it is always helpful to involve them early, as they're the ones who will be doing the work.
Prepare, prepare, prepare.
Once you've brought your team on early, make sure to include them in the conversation regarding shot design. Loop them together with your DP, production designer and your sfx team to talk about how things are going to be done, who will be responsible for what, etc. Your set builds, lighting setups, practical sfx setups and vfx all have to work in concert to get the look you need, and everyone needs to be on the same page. The more synergy there is, the less budget surprises you'll have in post.
Concept art helps inform everyone at this stage, not only to visualize what you're talking about, but to help push the conversation forward regarding best methodology and exposing hidden gotchas that are design dependent. If you have the budget, and you're working on a vfx heavy project, then previs is your friend. Previs (or pre-visualization) is your chance to figure out exactly what your shots will look like; it's not just a "we'll kind of make it look like that" proposition. If you can, integrate accurate representations of your sets/locations into the previs. Make the scale work, get your camera moves working, design your shots here. The more you stick to your previs, the more your vfx needs for those shots stay a known quantity.
To a certain extent, no matter how you cut it, visual effects can be expensive. It's very labor intensive, and it's being executed by a lot of very smart people. When you're shooting, it's easy to see what makes the days cost what they do. Everyone and everything is right there for you to see and watch the work happen. With vfx, it's very easy to forget the army of talented creative and technical minds that work on your shots. Though some of your costs will be fixed, the preparation you put into vfx planning during pre-production is one way you can help control them.