I live in a small suburb, so our pay rates here may vary, but I tend to find that a summer nanny would cost more than just putting your kids in a day camp if they are older. The day camps in my area are ~$1,000 per month for full day, and nannies are probably closer to $2,000 per month.
It really just depends what you want your kids to do all summer. With the camps they have to wake up early and have a very structured day, but they have an endless amount of activities to choose from (the girl I watched did gymnastics, tennis, dance and one other activity all in one summer), and I think it also helps them retain information between school years. With a nanny they can sleep in, relax and have a choice of whether they want to just play during the day or go on a special outing.