Depends on the country and system and other's have pointed out here that in the USA adoption is nominally free. I know my friends spent many months initially onboarding the process and went to countless interviews and did multiple health screenings and had to hire experts to write justifications for a cronic health condition that one of them had.

Anyways they were rejected. They went to a different system and I don't know much about the cost. I do also know they were eventually rejected on citizenship grounds. They moved back to their home country and I don't keep much in touch with them. I know they still haven't adopted and last I heard they were still trying.