How much would you pay your babysitter?

My son will be 5 months old when I go back to school. I will be looking for a sitter to watch him during the week when I'm there (about 17 hours) plus the time it takes me to get to and from school, and probably one night a week for a few hours. I'm thinking about 25 hours a week. I want to hire someone who is 18 or older and who has at least a year's worth of experience with a child so young. How much should I expect to pay weekly?