a)
He doesn't understand economics. The smartest people, in the world, in any one discipline, will often be handicapped in others. One of the two main philosophers who influenced Hitler made this point perhaps best. I know someone well that's no Perelman, but more mathematically advanced than most of the rest of us. Alas, I understand economics better than he, because he's focused conceptually on space-time stuff, flows of electrons, etc. It's not currency. It's not brute mathematical strength, but orientation - a direction turned.
b)
Integrity. Like Sartre, he doesn't want to capitulate to the organization giving the prize. Unlike for most of us, accolades negate objectives. Cedeing to the award allows the institution to claim him as object. "Please help us in welcoming Academy Award Winner, Grigori Perelman!" [Perelman enters hammy-smiling, irrespective of mood, offers platitudes to the organization, validating its structure, objectives, and forwards to say what they want to hear about what
he's thinking and doing.]
c)
Money is demanding. The old truth about one of the primary occupations of the wealthy is preserving wealth is true. The rest of us think it means you can go on holiday at will. You can. But you'll probably spend part of it thinking about your wealth, or will again as soon as you get home.
d)
Simply bureaucratic, perhaps accepting the award requires more than just the words "I accept" (I don't know), and he doesn't think whatever efforts are worth the benefit. He may have had to complete a report, sign a contract, etc. "I know how to control the Universe. So tell me — why should I chase a million?" (translated) In addition to the other arguments, the word "chase" sounds like there would be still more work to do for the prize.
e)
He's a committed communist. The only use a communist would have for $1 million is a war tool. Otherwise, while potentially useful, it is not in-and-of-itself, and the ideal is that this is the way it should be.
