What should I pay my babysitter?

I’m stuck on what I should pay my babysitter, here’s some background info:

She’s one of my best friends, absolutely LOVES babies and willingly told me she wants to watch my son for free while I work to control her “baby fever” and to enjoy time with him... she already comes over twice a week for a few hours each time just to hang out with him already lol, but obviously I want to pay her now

My baby will be 3 months old when that time comes and he’s an easy baby, I mean he never cries, easily soothed, easy overall

I live in upstate New York, minimum wage is $9/hour

As far as our income, my boyfriend makes $1500/week and I work as a waitress and generally make at least $100 in 4 hours, so she would only be babysitting for those 4 hours I’m gone, I also live walking distance from my work so I have a zero commute time too!

I’m thinking $40 a shift? Is that too cheap? Comes out to about $10/hour... sometimes I’ll be out earlier than 4 hours, other times a little late, all depends!