How to charge for Freelance projects?

I’m a copywriter with 4 years of work experience with some fairly big brands. Revently a few people have been reaching out for copy projects. Some are willing to pay per project, and others look at a long term commitment. So far I’ve been letting them tell me what they are willing to pau and if it seems okay to me I take it up. I want to ask if there’s a better way to work around this? How do I know what’s a fair compensation to write a video script? And what do I quote when someone wants me to come on board their brand for long term projects? Please advise..