The winning candidates in each of the last few US presidential races have headed campaigns in the course of which about $1,000,000,000 was spent. There is no reason to suppose that the winner of the 2016 race will ride any smaller a wave of money.
Donald Trump claims to possess a personal fortune of $10,000,000,000. This claim is unlikely to be true. More to the point, whatever the true scale of Mr Trump’s wealth, very few businesspeople are in a position to liquidate 10% of their holdings in order to finance one personal project. Mr Trump’s debts and other commitments are such that he is surely not at liberty to do that. Estimates of Mr Trump’s cash on hand range from $70,000,000 to $250,000,000, far short of the amount that is typically spent even on winning a major party’s nomination, let alone competing against the nominee of the other major party in the general election.
Mr Trump continues to assert that he has enough money to self-finance. His refusal to solicit campaign donations is so essential to his appeal that it is unclear how he could start asking for money without dynamiting his base of support.
That creates two problems. First, Mr Trump’s campaign expenditures thus far have been quite modest. He has received so much coverage free of charge from cable news and other media outlets (all the way down to this blog post, apparently) that he hasn’t needed to buy advertising. The only way he can keep gaining that free coverage is to make news, and the only way he can make news is by making remarks that are more shocking than any he has made before. Unless conditions turn so bad that the electorate starts looking for an out-and-out revolution, that’s a one-way street that leads directly to a brick wall.
If Mr Trump somehow manages to be elected president, he would face a second problem. Assume that the net worth of all of his assets really were as high as $10,000,000,000. And assume that he was able to sell them all at their full value, despite the fact that every potential buyer would know that that he was under pressure to sell them. Assume all that. A US president is effective only to the extent that s/he is the leader of an effective party. If Mr Trump has $10,000,000,000, it might conceivably be possible for him to spend $1,000,000,000 of that and finance a successful campaign for the presidency. But even $10,000,0000,000 would not be enough to finance the entire Republican Party for four to eight years. Presidents help their parties raise money. They are expected to do it. If Mr Trump should refuse to do that, he would quickly lost the support of his party and with it any chance he might have of enacting his platform.