Yes you will need to get a lawyer to get get the custody of the child switched over to you. My wife and I tried this about 4 years ago. After spending the weekend with the baby in the hospital, an hour before we were set to go home, the mother changed her mind. So I wish you luck and hope and pray you do not have that happen to you or anyone.
As for the costs, I have a good friend that is a partner at a law firm that pratices family law as well as other things. He was pretty much working for free, so our retainer was only $3k, and he said if all things go smooth that is all it should have costs. Court costs alone were close to $1,500. Luckly, he was able to get the largest portion of the money back for us since we were not pursuing another adoption at the time.
Let me know if you are interested in his info.