I would be honest and say what you said..they're your top choice of where you've been accepted, but you are waiting to hear back from a few other schools you've been waitlisted at...at which point you will be able to assess the best financial option for you. That way it sort of takes them out of it and makes it about finances too. Didn't you say you were going to hear back from your first choice waitlist monday (today?) How did that go?