How much should I charge?

I have recently met someone who wants to pay me to help her out with childcare. I'm new to doing this so want to make sure i dont over or under charge her. Here's the arrangement: I'll pick up her daughter from day care at 6, take her to my home and watch her until 830, when her mom picks her up. In that time I'll make her dinner and one snack. The daycare is approx. 23 miles from my home so about 50 miles drive to the daycare and back. How much should I charge for this service? edit: I am in Orlando, Florida