As I don't practice in New Mexico, where I assume you will file the step-parent adoption, it is impossible for me to give you an actual figure. However, that being said, attorneys generally bill for this kind of work in two ways - either flat fee or by the hour. The most important thing here is that you go to an EXPERIENCED adoption attorney. Many attorneys will tell you they can represent you in this type of case as it does not sound like it will be contested. But, you need someone who knows exactly how to proceed with your step-parent adoption. If you hire someone who is experienced, you will keep costs down. In the Washington metropolitan area where I practice, a step-parent adoption that is not contested (extremely important distinction) would probably cost several thousand dollars. You can find an experienced adoption attorney through the American Academy of Adoption Attorneys.