Baby sitting

So this girl I used to work with had a falling out with her dad, who watched her kids. I’m currently a sahm to a one year old. And she has asked me to baby sit her 4 year old and 3 month old. She would like to pay me monthly, she paid her dad 300 a month to watch them. But I’ll only have them maybe 2-3 days a week. So I don’t wanna sound over pricy so I need an idea of how much I should charge?! What would you pay a baby sitter for 8 hours a day. Couple times a week. Monthly rate.