High school sitter gets $8/hour. Her parents (friends of ours) told us that that is on the high end and if the house isn't spotless when we get home, we should let them know.
College sitter gets $15/hour. We are overpaying her too, but I know that she could earn close to that with an on-campus job. We all adore her and think she is worth it, especially because I have a child care subsidy grant through my employer that offsets some of the cost.
We never have these sitters babysit after bedtime, but if we did I would try to negotiate a lower rate for sleeping hours.
I have one active, non-napping 2-year-old.