Can I adopt an 18 year old child(who lives with me now) in Alabama? About how much would it cost me?

Yes, this could be possible in Alabama and should be a fairly quick and simple process if both parents and the minor are willing to consent. Contact an experienced adoption attorney to handle this matter for you. Consider the following disclaimer in regard to the comment above: No representation is made that the quality of legal services to be performed is greater than the quality of legal services performed by other attorneys. Further, the response given in this forum is not intended to create, nor does it create, an ongoing duty to respond to questions. The response does not form an attorney-client relationship, nor is it intended to be anything other than the educated opinion of the author. It should not be relied upon as legal advice. The response given is based upon the limited facts provided by the person asking the question. To the extent additional or different facts exist, the response might possibly change. Attorney is licensed to practice law only in the State of Alabama. Responses are based solely on Alabama law unless stated otherwise.

Yes, but lawyers can't quote fees in this forum.

You are going to have to get consent from the parents or terminate their parental rights. This is probably actually pretty complicated and you should call an attorney in the area.