You could also try to put together a budget based on $10/hour, minimum wage, and a 3rd higher rate. Seeing just how much money your potential nanny would be earning might help you decide what a liveable wage is. (Don't forget to include OT after 40 hours, and take some off the gross for taxes.)
Also, look at YOUR budget. How much do you want to pay for childcare? How much can you afford to pay? Often, if you meet in the middle of those 2 numbers, you find a good compromise.