Congrats!!

You can always say something like "Let me write you a project quote and outline the potential costs to you". Then you can go back on your on time-never on the spot- and try to figure out your expenses.

I am a visual artist not in film however but the idea is the same---Get an agreement in writing! And asking for half the pay up front is standard.

Maybe this will help?
What should I charge? | Videomaker.com

FREELANCE CONTRACTOR AGREEMENT -- FilmContracts.net