i don`t think $18,000 IS ANYWHERE OUT OF THE RANGE. but the final cost should vary on both sides of this price. last time i replaced joists on the east coast in the 80`s, i remeber the price as being between $20 to $50 per joist. insulation shouldn`t run more more than a few hundred. the remainder varies on the complexity of the effort involved. i can`t say enough about being cautious around molds. i`ve read they're as dangerous as lead if the proper precautions aren`t taken. the joist dont have to be replaced, but maybe could be sistered on to. the mold needs to be properly/safely removed. then a proper enviro. reestablished under the house. reinsulate and patch any damage above due to the repair efforts. in a house that means so much to the occupants, it shouldn`t be too much to spend if the funds are available. hope you make a balanced decision.