It's not money, according to McClellan's publisher, Peter Osnos. From the WaPo:
[Osnos] also dismissed suggestions that McClellan is merely hoping to cash in. Unlike some larger publishing houses, he said, PublicAffairs almost never pays more than a five-figure advance. "No one has ever done a book for PublicAffairs for the money," he said.
But wouldn't a low advance increase McClellan's incentive to make some money off of his book's sales--which, in turn, could lead him to write a harsher book than he originally intended (which is what, according to the same WaPo article, indeed happened)? After all, Ari Fleischer was free to write a bloodless account of his Bush years that bombed in bookstores because he'd already been paid a reported $500,000 advance by his publisher; the fact that his book didn't sell was no skin off his back. If McClellan hopes to make any money off of his book, he's going to have to do it the old-fashioned way: from sales.