At MIT you're not just paying for the education, but you're paying for the network of current students, professors, and alumni. You're paying for name-brand recognition and an easier way to get into grad schools and the job market.

I would look into financial aid at MIT and speak to financial aid advisors who work there, to see if you can get some sort of help.

If I were in your shoes I'd go to MIT, but then again I don't know what School Z is (i.e. if it's Carnegie Mellon or the University of Richmond, to name two very different schools).