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.
Involve your vfx team now.
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.
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.