Our son came to us when he was a year old in 2009 via domestic private adoption. The cost ended up being $19K, which was a combination of the Home Study, advertising, legal fees and travel expenses. The cost would have been the same for a newborn or for a child a few years older than our son.
More recently we learned that our son has a little sister, who was already in foster care. As her kin, we were offered the chance to adopt her, which we're doing now. The total cost of this adoption look like they will be about $1,500, which is basically our travel costs to go to the state where she was in foster care to bring her home. In the meantime we're actually being paid a small daily stipend, and are receiving a contribution towards the costs of daycare. To adopt from foster care we had to go into the foster care system ourselves, which is what we did. I don't see why you would need an agency to adopt from foster care. Keep in mind though that foster care is more about reunification with the biological parents, or their kin, first and then if that's not an option, the child may be placed for adoption with the foster parents likely being presented with the opportunity.