If your goal is to write, direct, and produce your own films, then there’s no point in writing something you can’t afford to do or something that could require a hefty fundraising effort. This, obviously, depends on your skill-set, experience, and your ultimate ambition. If you are a VFX master, then you could and should definitely make a VFX-heavy film, but if you don’t know a thing about VFX, then what’s the point of writing a movie that depends on it?
Writing a movie that you can afford to make tomorrow will make your life a whole lot easier today and will increase the likelihood of your movie actually being made once the script is complete (even if you only have $300 in the bank).
The Grand Budapest Hotel had a budget of 25 million dollars. Is this a dream budget for many young directors looking to get their film made, yes. However, in Hollywood, this is considered a very strict budget. Take a look at how Wes Anderson and his crew worked around their own budget constraints and won an Oscar for their art director.