Rune_Walsh wrote: If he didn't want to keep a cent for himself, I just don't understand why he couldn't give it all to charity. It would be an incredibly honorable thing to donate to people in need, schools in need, cancer research, etc.
It isn't hard to imagine reasons why he didn't want to go down that road. Let's suppose a prize winner has a number of friends and relatives who aren't exactly on the poverty line, but could certainly use the extra cash - like most of us. Some of us are too proud to accept financial help, but not everyone feels that way. If he accepts the prize and immediately donates the whole sum to charity, it may still create resentment. A friend may wonder: "He knows I'm struggling to pay the rent and get my son through college, but he won't give a cent to me." Yet once he starts giving handouts, the floodgates open; if he can help a friend, then why not his impoverished cousin, or his next door neighbour, or the milkman, or the university at which he did his degree? So he opts to decline the prize - not just the money, but the prize itself. But all of this is just speculation; my point is that he probably has his reasons.
According to a potted biography available online, it isn't even the first prize he has turned down. He evidently does not wish to receive any accolades from the mathematical community, regardless of the form it takes. In the past, he has stated: "I'm not a hero of mathematics. I'm not even that successful; that is why I don't want to have everybody looking at me." We don't know the life events or circumstances that might have shaped his attitudes. Without that knowledge, it is easy for us to misunderstand him.
The Clay Mathematics Institute, which awards the prizes, is a privately funded organisation that runs educational programmes and offers support to students and academics in the field of mathematics. As such, I am sure the money that Dr Perelman has turned down will not go to waste.