K.V.
I am curious about that, as well. I've tried asking around...it really seems to depend on the age or the sitter and their experience level, as well as if it's their main source of income. All the rates I listed below are per hour.
It seems for a high school student, the rate is anywhere between $5-$10 (as low as $5 for one child, then add more $$ for additional children - but some people pay $7 for one child). For college students doing this as a summer job, I asked a girl at church what the rate tends to be - she said $10-$12 is the typical rate. For an adult who is doing it to help, really whatever is reasonable to make it worth their while - most of them will be willing to give you a rate. (A lot of teenagers are afraid to give a normal rate - so you can always ask their parents in that situation.)
I'd say continue to ask around, hopefully you get more responses here, too! If all else fails, be honest with the sitter and explain that you have no idea what a fair rate is - I'm sure they will be reasonable and let you know what is 'normal' for them or what they'd accept.