Personally when I am on vacation with my family I am there for my family. However, why would be want to cheap out on te most precious thing in your life?
$20 per hour seems reasonable.
Its true you can get cheaper.. but I would wonder what the background of such sitters are. It is our kids you know that we are entrusting with strangers. Definitely high on the list would be a referral from a trusted source (i.e. hotel), bond/accreditation, etc.