How much should I pay my baby sitter?

My son is 7 months old and I pay a friend of mine to watch him for me on average about 7.5 hours a week. I have a low income right now despite working multiple jobs and I am also going to college so I am tight on cash. I want to be fair to her though and pay her appropriately while staying within my budget. I figure 7 hours is approximately the equivalent of one full day and for one full day I would probably pay abut $40. Does that sound reasonable?