How much monthly does a 5 year old cost?

Lanae🚼#✌️ • Mom of two 💙💗 Last of the bunch due May 2018 My ancestors had to shutup, I don't 😎
So my SO and I are trying to figure out what amount would be a good amount to send to his sons mom. He is not on child support and they share custody. His son visits twice a month. And most of the summer. Even though the mother and I don't get along I know it's hard to raise a kid soley on one income so I want to make sure she's comfortable.