The Feast: Given the possibility of food allergies, staff coordinates meals. I am fairly certain that bringing a dish is not required (I don't recall bringing anything last year) but send a note to staff if you are so inclined to share.
Cost per person: Don't know.
Sleeping facilities: The three cabins, where it sounds like people will be sleeping on Saturday night, are heated with spring bunk beds, a bathroom, and showers. A sleeping bag is typically all you need.
Costume: It is certainly ok to have a "mostly done" costume. Everyone understands that costumes evolve, get tweaked, rebuilt, and abandoned. The only issue is that armor is initially "graded" by staff for purposes of determining its number of armor points. If you swap out for new armor, let staff know so that they can re-grade it.
Hope that helps.
Bartimaeus