I used to work in a childcare centre and picked up a few babysitting jobs from there.
I charge $15 negotiable per hour for one child ($5 extra per hour per extra child). I've found that this is a lot less than other people I've spoken to about it and that the people I regularly babysit for often insist on giving me more than that. Not that I ever would expect that from anyone.

Paying extra for quality doesn't always work, I get asked repeatedly to babysit by the same people and they often recommend me to their friends(so I must be doing something right).

ETA - I am 21 though so maybe less would be appropriate. Although I think the skills are what should be rewarded not the age of the person.

HTH