Why does Black Jack charge exorbitant fees?
I don't believe there was ever a solid statement in the Manga for Black Jack's high prices. However recently I came across two interesting stories in Vertical's volume #2 that might shed some light on Tezuka's intent.

In the story &quot;Granny&quot; , which is about another doctor who also charged a lot and an old woman who worked her whole life to pay off a debt to the doctor, Black Jack says


&quot;I thought I was the greediest man ever.&quot;


This seems like kind of a joke, so I don't think his prime motivation is greed. In fact Black Jack doesn't seem to spend much of his money ( he basically lives in a shack).
Later in the same story he charges the son of the old lady a huge sum, but that seems to be a test to see if the son is willing to make the same sacrifice that his mother did. It's not clear if Black Jack actually charges the son after the son passes this test.


In the same volume there is a story of Black Jack's recovery from his surgery as a child when a classmate donated a piece of skin to save his face. Black Jack   Promises to become a doctor so he can pay this friend back.


These two examples could provide some reason for his fees, but I tend to think that he really just decides the fee based on the character traits of the patient, circumstances, or finances of the requestors to either punish, reward, or test their worthiness.