Woah that's pricy! Flight, meals and I'm guessing Disney passes? It shouldn't be that much for an extra child.
Especially for an eleven year old on a trip that is presumably already happening, meaning accommodation costs aren't going to suddenly shoot up, or car hire or whatever.
It wasn't ok to tell your DD without asking you, both money and respect for you, she's eleven!
I honestly don't think eleven is too young for a child to learn that money doesn't come from nowhere and can't be thrown around. It's a valuable lesson!
Maybe talk to the parent, they may have wanted company for their child and got overexcited (and yes inappropriate - unless the child is 18 and paying for themselves, the parent should talk to you first!), maybe they are just clueless about the world.
I don't think your DD will be especially disappointed. Its a sudden suggestion, not something that's been dreamt of and planned for months suddenly being cancelled. I think the notion will go as quickly as it has appeared.
It's fine to say "DD, trips take lots of planning to organise lots of things. I know you'd love to go, but we are going to do xyz and we can talk about Florida/Disney later and research planning a trip there another time, but we all can't just suddenly spend lots of money on a trip that we haven't planned"
Maybe add "Plus, have you had all your many travel VACCINATIONS? That takes time because there's so many!" Yes I know there are none, obviously, but that may be enough to keep DD in the country for a few years more:)