class Man {

   Set cars;

   public Set getCars() {
      return cars;
   }
}


class Car {
   Man owner;
   double price;

   public double getPrice() {
      return price;
   }

   public Man getOwner() {
      return owner;
   }
}

Double result = (Double)sess.createQuery(
    "select sum(car.price) from Man m " +
    "join m.cars car where .....").uniqueResult();