Seasoned Nanny here.
It has been my experience that it is better to set a weekly rate. I've also found by what most families want to pay, and what other nannies with whom I've spoken make, that most seem to pay $600-$700 week for one child. I have been blessed and have never had a position paying less than $700, and currently make $900/week. My highest paying position was $1200/week, but this was a share, so a little different.
I admire parents who realize that they pay for the care of their treasures, their children. This, at least it is for me, is a chosen career. I'm not a babysitter; I'm a professional who is passionate about children, and thus, I believe I should be payed as such.
That said, my advice would be to pay your nanny what the job is worth. Ask yourself, what would someone need to pay me for me to invest my time, my effort, my passion and my joy into spending most of my day with someone else's child. What you would have someone pay you, you should pay.

Best of luck to you in finding a great nanny. Hope all of us have shed a bit of insite...