I babysit very, very often, and it depends entirely on the region, age and education/experience of the sitter, and how many kids need sitting and how old each of the kids are.
@SuperMouse is getting a great deal, which you would never get around here (CT). I typically ask for 8$ an hour if it’s 1–2 young kids who will be asleep most of the time. I babysit for two families at least twice a week, and they both started at 8$ an hour and gradually increased to about 12$ an hour once they began to rely on me as their only sitter. For families with more than 2 kids and when the kids are older, I ask for about 10$ an hour, more if the kids are generally awake the whole time I’m there or if I have to make dinner, bathe them, put them to sleep, etc.
(I’m also 18 with 4 years experience working in a daycare and am known around my neighborhood for having a lot of experience with kids, so I could easily get away with charging up to 13$ an hour starting rate for just 1–2 kids, but I personally feel that’s too much).
Hope this helps.