The going rate is approx $10-12 for a singleton and for twins it's at your own decretion. It really depends on what you are comfortable with and experience. I would say no more than $20 but tipping is ALWAYS appreciated.
I am also a mother/nanny/childcare provider/babysitter/certified medical assistant/certified night nanny and Always available and know what it's like to try to find the perfect one for you and your kids.
Good Luck and I hope this helps. You Always want to make sure that they can keep up w/your kids.
M.