I have experience with a wide variety of scriptwriting for different clients since I began freelancing - including fiction, commercials, ADs, promos, explainer videos, VSLs, how-to videos, whiteboard videos, ESL course videos, medical scripts, and scripts for other corporate projects. Once I understand what the client wants the script to accomplish - I can find a way to achieve it.
Typically I'll quote a fixed price to write the script once I have the details.