NANNY: how much should I charge?

I am going to be a nanny of a 6 week old baby in Tulsa, Oklahoma starting the beginning of October. How much should I charge for 30hrs/week? It would be in my home.... and if i add her older 4 year sisterhow much should I charge( the 4 year old will be starting school in the fall rite when the baby is born so I would only have her on fall break/ winter break etc?)