Financial Aid in graduate school is a tricky thing. There are various definitions the school uses for it and there are various ways to get it.
Financial Aid can be offered by the School or by the Department. The school usually offers a complete tuition waiver or atleast a partial in most cases.
Departments offer Teaching Assistantships (TA) or Research Assistantships (RA). These involve the students into either teaching assignments or into lab work respectively.
A GRE score of 2100+ and a good resume can be plus points in securing stipends.
The program one applies for also decides the stipend availability. PhD programs usually pay. MS is like a short term course which does not pay back the university in terms of original contribution to research. Hence stipends are less frequent for them. Also computer science courses have a dearth of available financial support. Biological Sciences still offers a good stipend support.
As for MBA schools. There is no reason for the school to provide financial relief to the student unless the student is exceptional and will contribute to increase or atleast advertise the school by becoming successful.
THE LOGIC IS SIMPLE. YOU HAVE TO CONTRIBUTE BACK 10 TIMES MORE THAN WHAT YOU GET IN TERMS OF STIPENDS.
IF YOU MAKE A BREAKTHROUGH IN RESEARCH YOUR DEPARTMENT AND YOUR BOSS GET THE PATENT FOR IT. MAYBE EVEN THE NOBEL PRIZE.
UNIVERSITIES IN USA ARE NOT RUN LIKE GOVT. COLLEGES IN INDIA. SUBSIDIZED EDUCATION WITH THE STUDENT NOT EVEN PAYING THE LIBRARY LATE FEES EVEN IF HE BECOMES A MILLIONAIRE LATER.