I use younger babysitters (12-14) and think you can pay them a little less. And I know my kids are easy to babysit, but if they were hard, it may be different (or we probably wouldn't ever get sitters).
I used to leave rules sometimes... mostly just when bedtime was and what they needed to do before bedtime. My oldest two are 7 and 5 and they can tell the sitter all of that now, so all I leave is our number. Also, 99% of the time we get a sitter, my kids are sleeping more than half of the time (or supposed to be). So I take that into consideration also. They just have to sit out my house once the kids are in bed.
So all that being said, we generally pay $2/hr/kid. I personally think making $20-25 for 3 hours is good for young babysitters. Especially because they usually LOVE babysitting and have fun with the kids... and only for an hour and then they're in bed. I have to say... we do have 4 kids (though I usually put the baby to bed first, so I just count 3)... and it would be harder to pay $2/hour for only 1. I think when we had 2 kids we did $5/hr.
And yes, I expect the house to be as clean as it was when I left! But I will teach my children to leave the house better than they found it.
I don't tell them to clean, I just assume they would. And if they don't, it's not a big deal--but I would probably pick someone else over them if they left my house a mess!