Oh man that's tough. Is the 100k just for tuition? There is also the cost of living in New York City to consider (maybe you already live here, but if not, its brutal!).
I am sure you're paying attention to what's going on at Columbia right now, there is a lot of tension in leadership and according to Democracy Now the percentage of students accepting their admissions offer is down this year. Thats likely for undergrad and may not be relevant to the MFA program - but if Columbia is what you want, you may have more leverage than you think.
If you haven't already, maybe just be aggressive in asking for money letting them know you can't otherwise accept, which sucks, but the worst they can do is say no. (I have a friend who negotiated a full ride at UPENN after not being offered anything - its definitely worth a try!).
And who knows, if they say no and you decline, maybe next year you get into a fully funded program - go with your gut!