I never really had a set fee but I remember paying one of the girls like $40 to watch Sam & Ben (Sam was 9, Ben was 2 1/2) to basicallt just watch them so we could go to a Chrsitmas party...and I didn't leave any actual rules other than to call us if she needed something...now I use my DD (19) and have been for awhile, but since we pay her to get her brother and sister up for school, she is also expected to watch all of them when we want to go out (generally about once a month) and she does not get any extra
It is part of her "rent"