Can you ask her what she usually gets paid by other people? That will give her a chance to let you know if she expects full pay or is doing it mostly as a family favor. (a spring Saturday is a big gift from a high school aged kid, though!)
We pay our teen babysitter about $9 or $10/hour, but if it was a full day I might ask her if she would take a lower hourly rate that day. For 7.5 hours, I'd probably offer $50. But I'm not related to her, and I have the idea that babysitting rates are really variable by area (they are in high demand here!)