Babysitting rates?

So I have no clue how much to pay a babysitter. We have a neighbor girl whose 15 who we may potentially have come over after school for 1hr so we can go to the gym. So 4hrs a week. They told us whatever we feel is fair? I dislike that response so much lol I don’t want to be too cheap yet I can’t afford a ton nor can I justify a ton after we pay for the gym monthly. I’m a stay at home mom so we have a limited income. Anyways how much would you pay a sitter for an hour?