Time is often the key factor, quotes for VFX work (like many other jobs) are generally done on time (applied to a day or hour rate).
Big movies involve hundreds of thousands of man hours, spread across thousands of workers.
On a TV show with a fast schedule it's important to identify things that require extra time as early as possible, that's something a producer should be doing, and consulting with department heads as and where necessary.
Speaking from personal experience, it's the pinning down of the "what" that can be a problem. Once the client knows what they want you can start to work out the "how" to achieve it.
If they can't afford the "how" then they can either come up with an alternative shot, try and find a cheaper supplier, or accept that something won't be great.
The audience may not be prepared to go along with that.