Think wheather you want to be paid a flat rate, per word, or hourly.
All I can think of is hourly, which would be $25-30/h. You could ask them how this would translate into the task required. It looks like they might want to pay you per piece.
You say "editing", but what you actually describe seems "writing". So which one?
I'd be honest with them. Tell them you normally charge $25-30 /h (or whatever you charge) and that you are not sure how this translates into this kind of work. They will let you know, and then you can see if what they offer seems fair.