That sounds about right; like other have stated, it depends on her age and "training". I have two kids, 3.5 and 2 years, and I pay my sitter $7/hr. She is first aid and CPR certified and a collage student. I pay her on the lower end because she is a friend of the family and offers to do it free for me. I also have a child with severe food allergies, so I'm pretty picky who sits my kids. I do have another sitter that I pay $10, again, just because I'm so picky. I was told by my friend of the family the "going" rate was $4-6/hour/kid.
If you have a sitter you like, just ask her if that's okay, and be open with her with the price. Oh, I also pay my sitter extra if she is there past 11pm and for gas money. A good sitter is priceless in my family though :P