Hi Dreu,
You will find that a lot of corps now charge for Open House. For us, it's simply a way to help offset the administrative costs it takes to run the event. If prospective members stay on with us, money paid goes towards their dues.
We use "auditions" as a way to help place people on parts, particularly in the horn line. For percussion and guard, it is more like a true audition. Auditions also provide a way for members to really take ownership of their spot. There's a bigger sense of pride when members know they had to "work" for it. I do want to emphasize that no one is ever cut from the corps. We find a place for anyone who really wants to be there. As you know, opportunity lies around every corner.