For an MBA, you should not even compare IIT Bombay and IIM Ahmedabad or Bangalore or Calcutta. The IIT Bombay is a prestigious engineering school. For management, IIM Ahmedabad, Bangalore, and Calcutta are the same. Both are in a completely different league. 

Considering only ROI makes no sense from the perspective of an MBA because an MBA is more than just an investment in yourself. These IIMs have much better professors, a world-class curriculum, peer groups, alumni, placements, student exchange opportunities, and so on.

So, if you get through them, please go for the IIMs. Financing your education will be simple because once you graduate from one of these IIMs, banks will be eager to extend loans, sometimes even without a guarantee. And if you do well, you will be able to find a good job and pay off your loan in a few years.