Payroll service for over the table nanny pay?

We live in El Cerrito and plan to have a nanny care for our two children (ages 3 and 1) this fall.  What is the going rate for a nanny caring for two children in the Bay Area?  Websites we consulted seem to be too low and inaccurate.  Also, we wanted to pay our nanny over the table.  Does anyone have any experience with payroll companies? Any recommendations?  Thank you!