In the past, it would not have been a problem because of the venue. Had they gone to the zoo, for example, I could have just driven to the zoo and paid my own admission so I could accompany her. I wouldn't need to be an "official" chaperone. I'd be a de facto chaperone. Fine with me.
But this was a play in an auditorium with obviously limited seating. No more seats means no more seats. I couldn't just follow behind in my own car, pay admission and join the group. We had other concerns. As I said, it's a very high crime area and I didn't know any of the parents that well who were chaperones. If it had been a close friend and I could have said, "___, do you swear to watch her like you would your own?" maybe it would have been different.
Last of all, we're talking a 1 hour ride each way into a city of a few million people. Vey busy highways. They hire very nice coaches with plush seats, restrooms and TVs in the seats. A far cry from the yellow school buses we took everywhere.

However......and I cannot believe this.....there is no promise that every coach would have seatbelts.

If I won't put her in my own car without a seatbelt, why would I allow her to ride unbelted on the interstate into a huge city on a coach with a driver that I can only hope has been thoroughly checked out by his employers?

(Recently there was an article in the paper about this very thing....the lack of verification of training and background checks on these drivers.) Since the schools get horsey about us driving them on our cars and not on the hired coaches, that was just one more issue.
We just said....Forget it. An amusement park sounded like a better day all the way around. Thanks for the support!
More on topic, I agree that BAB is great about keeping kids on budget. At ours, after the kids select/stuff their bears, they all sit down and each one is told how much they have left to spend on an outfit. They get a tag or something to that effect and the workers help them pick something out if they get stuck.