Another adult: 1/3 of your living expenses. Assuming you in the same living arrangement.
Another child: more expensive than college

Say you are a single person making $50k. How much would your HHI need to increase by to cover another adult (spouse) and each child, and maintain a comparable but realistic lifestyle? For example, we travel less as a family of 4 since tickets are expensive and its hard traveling with two small children. So I considered our travel expenses to double when the household went from one adult to two, then it stayed flat despite two kids. Lets assume one spouse stays at home with the kids since childcare costs vary so much.