The money comes from people who have been (or are) buying/pumping Steem. But why should someone buy Steem? What is the incentive of a reader (upvoter) to buy Steem? If you open an account you get some free Steem Power, but there is no need to buy more Steem. If there is no need to buy more Steem, what will happen with the price? Is this system sustainable? I think this is a very interesting discussion and time will tell. Also Steemit is in beta and a lot things can change in the future. But I think you're right. The question: where does the money come from is not really answered in this post.