Why are adoptions so expensive?

I've heard that some adoptions (especially foreign ones) can run up to 20k-25k. At those prices not many can afford to adopt a child. Why doesn't the state subsidize some or all of the costs? The child wold be better off with a family and it saves the state 18 years of costs. It just seems to me that if the government really wanted to find a good home for as many kids as possible they would make it much easier for the families.