I do not think that Mr. Mankiw writes for money (I Can Afford Higher Taxes, But They'll Make Me Work Less. New York Times Sunday Business, October 10, 2010: page 3).
I believe that he writes to share his ideas with the rest of us. He accepts speaking engagements so that he can influence senior corporate executives and government officials. He speaks and writes so that, ever so slightly, he can influence the economic policy of our country. He writes so as to leave a name for posterity.
Money is merely a by-product of of his activity. His motivation is intrinsic. If the group making the invitation was influential enough, I am sure he would speak to them for free.