As a babysitter and now nanny, I know the rate varies greatly by location, number of children and age/experience of the sitter. I live in expensive Northern California and have made anywhere from $10-$15/hour. I am nearly always asked what I charge beforehand. In my own personal calculations, I am 22, can provide transportation, and have had about 10 years of solid experience with terrific references so I usually start my rate at $12/hour and add $1/hour per child.
care.com has a great rate calculator here:
Hope this helps!