Charge as much as you can. (I assume you write to make a living, not just for fun?)
Medium length answer:
Remember, you're the one with the necessary skills. If the person who asked you to write the script could do it himself / herself, they wouldn't need you. If they can't afford, (or, would rather not pay) professional rates, then you don't need them.
Do not, repeat NOT, charge by the hour, or by the word.
That simply puts you into a bidding war with competitors, and takes no account of whether you're competing with writers at your level of skill. Writing isn't a commodity, it's an individual talent. YOUR individual talent is what you have to sell.
Assuming your prospect is approaching the project on a business-like basis, they'll be looking at their scriptwriter options. You need to set yourself apart from the rest, so your focus must be on what you personally, can bring to the equation.
Identify your strengths, as they relate to this particular task, and SELL your uniqueness, to shift the focus from cost, and onto Value for Money.
If you don't treat your work as valuable, why would they?
Before any discussions about price, ask them what their budget is. If it seems like enough to you, jump in. Make sure, in writing, exactly what you're contracting to write. Equally, if not more, importantly, identify what you're NOT being paid for, (ie; revisions, rewrites etc), and formally state that you're fully prepared to do them, but they will be at extra cost.
If the budget seems 'kind of OK' to you, tell them you're interested, then prepare a formal estimate / quote, based on the points above, and negotiate the extra $$ you require.
On the other hand if their answer to the budget question is "We have no idea, that's why we're asking you", walk away. That's not the kind of gig you want to spend your time on - it will only end in tears.
I'm planning to write a Hub on "How Freelance writers can increase their income, by shifting the focus from cost, to value." (That might not be the final Hub title). It's an entire process, based on my own experience as a successful freelance copywriter. Should be up in a week or so.
You might get something out of it, if you want to keep an eye out.