As for the $ issue, my DD's friend age 14 paid for her airline ticket. I paid for the hotel room, as it would have been the same price with or without her. But still, that was a good deal for her. SHe brought along spending $ for all meals and souvenirs. I ended up treating for one meal and buying the soda that the two of them shared.
I took them by myself, so one of my rules was that they couldn't ditch me the whole trip. I made that very clear to them, and reminded them also during the trip. Sometimes they would go on rides that I wouldn't go on, and I'd meet them afterward. I also gave them some free time at City Walk, with her mother's permission.