We use a combination of a holistic/zen approach...
First, did he/she drive themselves, or did we transport?
How long were the kids awake while the sitter was there?
Did the sitter have a friend or not?
Did the sitter make dinner or did we provide?
Is this an active, fun sitter, or a dull, bossy sitter?
Were the kids difficult or okay--they're never easy, take after their dad!
We love our sitters, and have the opportunity to choose whomever we want as we both teach highschool. We're really fussy about who we ask to sit, too. We generally won't use anyone not in high school, and prefer sophomores and up. Our favorites are the star couple, cheerleader and linebacker.
For our kids 5 and 9, who are normal-not-angel kids, we usually pay around 20-30 for an evening. We usually need a sitter around 4:00 pm and are home by 9:00pm. No sitter has ever refused to come back, and last Friday I could have had 4 sitters (we went with two, one for each child, and they took the kids and a picnic to the park, got the kids to bed, and created the picnic, they transported themselves). We gave each $20. We have taken the rising cost of gas into account, and pay a little more for kids who drive.
Keep in mind that Binghamton NY is very economically depressed, and we can't pay our sitters more per hour than their parents make! Also, our daily, licensed in-home care giver gets $20/day (7:30 to 3:15ish), so feel odd giving a high school kid more.