Most plumbers do not and should not make structural repairs. The type of repair, and the cost, will depend on the extent of the damage. It could cost anywhere from a few hundred dollars to a few thousand dollars. There's no way to tell without looking at the situation. I'd start by asking your home inspector about the situation and about what type of specialist he/she would suggest. Your home inspector has seen the damage and can advise you. Most of us are willing to help our clients with these questions.