In our area mother's helpers make about $5 an hour, and babysitter's $10 per hour.
You'll find that the norms vary a lot geographically. One thing to take into account is that the 2011 Washington minimum wage is $8.67. Those amounts in our area are below & just above minimum wage. If min wage in one area is $4... it makes sense they'd pay $2 or $5.
I've yet to have a middle school girl babysit... but as a middle schooler I babysat 4-5 nights a week (had more than 10 families that used me regularly. At the time (80's in california) I got between 10-20 per night averaging from 4pm-11pm. MY biggest problem was that every family had different food rules. Some said "help yourself" but didn't mean it, others DID mean it, others had allergies so you couldn't bring your own food, others expected you to bring your own food. It's irritating when something as basic as "bored + hungry" (after the kids are in bed) is something that you have to guess on. The other problem I had was late bedtimes and bathtimes. Kids are rarely in bed on time when they're being babysat, but at least they go to bed eventually in most cases. In cases where they're up until the parents get back, you stumble home absolutely exhausted. Lastly, buzzed fathers driving me home always gave me the creeps. Not sexually... but because I was NEVER supposed to get in a car with someone who had been drinking, but 9 times out of 10 I could smell it on them / they were all friendly (again not in a creepy way). But as both ADULTS, and as my EMPLOYERS... it made me feel very very awkward. I didn't feel like I could turn them down and call my mom (I was young and they were authority figures, and sitting around waiting, blah blah blah)... so I usually tried to arrange for them to call 10 minutes before coming home so I could call my mom to come pick me up.