Other than hiring a coach on PrepLounge, my sense that it's less about how much money you have and more about the connections/access you have a result of your wealth. In the past, MBBs (particularly McKinsey) at entry-level post undergrad studies used to be heavily about coming from the right socio-economic and educational background. Below are a few examples I've seen of successful McKinsey BA/summer BA candidates who have made the most of their priviliged background:
+ Target school and strong educational background that has helped them naturally acquire the problem solving intrinsics (e.g., critical thinking, strong communicator, intellectual curiosity, etc.) that is being tested in a case interview
+ Prestigious internship(s), especialy very early as a freshman due to family connections etc.
+ Interesting extra-curriculars as you they haven't had to work while studying and also have access to unique extra-curricular opportunities through wealth
+ Well connected with the right group of people (incl. through family connections) who are at MBB, often senior partners, due to being part of the same social circle