Needless to say, it was a bad idea to back out at the last minute.
Is she not able to meet the rent without you sharing it with her? If she is able, it's not quit that serious.
So her $1500 sounds ridiculous. Most college rentals around here charge by the head. So if you were to move in, your rent would be the same as the other 4 occupants. No more no less. You might check this out. If it was by the head and the landlord found out you would have been kicked out.