There are a lot a factors, in general if it's a shorter time I'll pay more per hour (I'm not giving someone $5 for watching my kid for an hour, but I might give $20 for watching my kid for 4 hours). It also depends on how many kids, time of day, if the kids will be asleep the entire time, do they need to make meals or change diapers, is it a one time thing or a reoccuring thing, how much do I want that person to baby sit for me again in the future, how well they actually performed, etc.

I evaluate the the whole gig and pay a flat fee based on a number of factors. Usually I'll have a number in my head before hand, but often that can be adjusted when I get home and we see how it went. I've paid anywhere between $4 an hour and $15 an hour it just depends.