The show sponsor has to pay for the facilities that are to be used, a set DCA judges fee, hotel stay for the judges, travel to and from the airport-hotel-show for the judges, food for judges, selling tickets & awards (if they so choose).
As for the corps who attend, it's all about the date of the show and the location. There's so many shows that historically always fall on the same weekend each summer.
The show sponsor then also can pay the corps, which will help draw groups. DCA pays each corps a tiny little share at the end of the season.
DCA has no risk in a show financially, their only risk is reputation. They don't want a show sponsor to show DCA in the wrong way, which is why there's a minimum as to how many corps have to be at a show.