I've got 4 kids and I usually pay 10-12 an hour depending on how experienced they are (and how outrageous my children are being). I don't see why they should get less/more depending on if a child is in diapers. It's all part of the job. However, if the babysitter is CPR certified, I do pay more, usually about $15 an hour. You'll have to see what the rates in your area are but be careful. On another forum I saw a mom asking if it was ok for a girl to be asking $20 per child per hour (this woman had 5 kids) so you could see how quickly that would add up. And the girl wasn't CPR certified either. Talk to your friends, see who they use and recommend. That's how I've found all of my babysitters, as well as through my Church. My Church has a great nursery program and it's run by our Church's Youth. The girls in there are totally awesome. The Church does pay them, but it's only $12 an hour flat rate, it doesn't matter how many little ones attend that day. Some days there may be one or two kiddos in there, but other days there could be as many as 10.
When I was a girl, I just asked for whatever they felt was reasonable. Sometimes I got very little and was frustrated because I felt my work was worth more. But other times, I got WAY more than I should have. The thing was, I got most of the babysitting jobs because I wasn't asking an arm and a leg. It was all word of mouth from the mom's I worked for which is why I said you should ask around your friends. Word of mouth is big because your friends will tell you exactly why they do or do not like a specific girl (or boy). I have never called a girl that was not vetted by one of my friends or that I had personally met before.