Summer Child Care Costs

My oldest son is getting ready to turn 5 and will be starting kindergarten in August. He has been attending the New Berlin Schools pre-k program and daycare for the remainder of the day. I am looking at options for him for child care this summer, outside of returning him to daycare for a full-time tuition since I have two other children in full-time daycare. I am thinking about trying to find a high school junior or senior to watch him at home this summer. I am looking for someone to care for him from 8 am - 5 pm Monday-Thursday. I need someone who can drive and we have a pool membership that the sitter can use to take our son. My question is what is a fair price to pay a sitter in this situation? Does any one have any experience in hiring this type of sitter? Any recommendations are welcome.