To me, this is very subjective and can be personal. Do you have it to be extra generous? If you don't, then anything you choose is the correct amount and you shouldn't fret or worry yourself.
If it were me, I'd give her a $20 ahead of time when we spoke & are setting up. Thank her for her help in advance, you know all the politeness. Theoretically she'll be running around trying to make your party perfect for you! Then at the end if she was awesome I'd probably giver her another $20 or so maybe.
All depending on party atmosphere of course, how difficult was it, was everyone polite? That sort of thing.
Have a great party!