This is always such a loaded question that is incredibly hard to answer. Unfortunately there are so many differing factors that can come into play that it’s impossible to just throw out an accurate number.
Agencies cost a significant amount more than doing private adoptions. With agencies they present you to expectant parents and help you match, and charge a lot of money for that service. There’s also the question on if the agency is ethical that is highly important to consider before ever using one. Agencies can vary drastically, 25k-50k+ per adoption.
Private adoption is what I did, and financially it can be more realistic. I self matched and all of my children’s birth families were in the same state as me, so there was no cost of travel involved, and minimal “birth mother” expenses. With private adoptions, depending on if you’re traveling out of state, helping with rent and other expenses, etc the range varies, but it can be done for 10k or less per adoption depending on support needed.