In terms of short term ROI, MBAs generally benefit those with a lower pre-MBA salary since the opportunity cost is lower for those folks.
There is a segment of folks who get huge scholarships for diversity purposes. As an example I had a classmate who came from Macedonia on a full ride. For her, the MBB post MBA at no cost was a huge ROI not because the pre-mba salary was low but because she paid nothing and upgraded to a T1 consulting firm, landing in the SF office during the boom years of tech.