No reimbursement from grad school for canceled open house flight expense
The starting point is probably to respond politely but firmly to the academic in charge of the admissions process, pointing out that you cannot eat a flight credit, and that the grad school's actions have led to a substantial and unexpected hole in your budget. Frame it as a 'moral' obligation rather than a 'legal' one. If you've had significant interactions with any of the professors as part of the applications process, you should cc them on the email. 

If you do not get a satisfactory response, you might then escalate matters by emailing the Head of Department/Dean/University President's office. Again, being polite and non-antagonistic but persistent is probably your best option. 

If none of this gets you anywhere, your options are probably to involve a lawyer and/or a journalist.