We pay $5/hour per child, so for us, it is $10/hour. They are there for 21 hours/week, so it's $210 each week, most weeks (if I work more, we pay her more, if I work less, we pay her less) - our sitter is a SAHM. We also will be paying her paid time off beginning the first of the year, in addition to paid holidays. This includes all meals/snacks etc.
As to how much of my income it is - it's significant. My gross weekly income is approx $360 - right around 58% of my income. We hover right around the line of is it really worth it for me to work....if DH made a little more, I could stay home.