How much do people pay babysitters these days?

I am the mother of a three year old little girl and two teenage sons. I hire a very mature 12 1/2 year old girl who lives two doors down to watch my daughter sometimes in the evenings, or when I need to run a quick errand while she is napping (my boys are far too irresponsible to handle the job, plus they have many activities and are seldom available anyway).  I pay my sitter $5 per hour, or portion of an hour. In other words, if she is here 3 1/2 hours I pay her 20 bucks.  Am I underpaying her?  None of my friends have children as young as mine (she was our little late life surprise) so I don't know the going rate, and I am certain rates have gone up since my sons were little. 

Although my sitter tells me the money I pay is fine, I know she would not say anything if it wasn't.  Therefore, I need some help here. Please let me know how much you pay your sitter so I can make sure I'm not underpaying mine.

Thanks!
Danine Manette
Ultimate Betrayal