I'd have a hard time paying less than $10/hour for such a responsible adult. And I don't mean, what the market will bear, I just mean it wouldn't feel right. In my area (Salt Lake City) nannies are paid $10-12/hour, going up from there, but $10 at the low end. I suppose a nice round number would work too, assuming the hourly wage isn't that far off from $10. So, if she worked a Saturday for nine hours (is it a full-day trip?) you could probably get away rounding down to the nearest $20 ($80). Around here anyways. And you may want to run it by her before you go. She'll probably say that's fine, and do it for you. If she thinks its too low, she just won't be available next time.