Yes, I am aware of fact that one person cannot give that huge amount of money himself. But I am also aware of fact that there are way of how to do it (create dummy non-profit organizations, and so on).
Non-profit organizations cannot give candidate campaigns money. They can only fund things like advertising that cannot be coordinated with the candidate campaigns. But of course rich people can do that directly. All the non-profit gives is pooling (more than one person contributing), which you don't have here; and anonymity, which you don't mention as a goal.
The easiest way for a rich person to magnify donations is to give to each and every campaign committee in all fifty states plus the national committee. They have much higher donation limits, and there are a lot of them. This works bests for major party candidates though, as the third parties don't necessarily have matching organizations.
Another trick is to have a business that gives out discretionary bonuses each year. Let it be known to your employees that you favor Jill Stein. Then increase their discretionary bonuses by the amount of any contributions that they or their spouses make. So long as you don't announce that you are doing this such that people can testify about it, this is difficult to prosecute.
But let's ignore the difficulty. Maybe you marry Stein. Then your money is her money via community property. So she can spend it all.
Hillary Clinton is outspending all the other candidates put together by a lot. It's ridiculous how little money Donald Trump is spending. Yet Trump is not losing by a landslide. In fact, he's winning as many states as Romney even though he's not nearly as competitive in spending. This gives a sort of upper limit to the benefits of spending.
Stein also has the problem that her ideology is most attractive to a minority of US voters. Basically left-leaning independents. Unless she would moderate her stances, she would have a great deal of difficulty beating Clinton among center-left voters. Stein also has no state or federal government experience. And no executive experience of any kind.
With enough resources, Stein might be able to increase her share of the vote from 3% to, say, 20% or 30%. However, this would almost entirely come from Hillary Clinton's share. The likely result would be better relative results for Trump. If Clinton and Stein each have 30% of the vote, Trump could win with 40%.
Another possibility would be an electoral college split. If no candidate gets 270 electoral votes (a majority of 538), the vote goes to the House. Republicans do and almost certainly will have majorities in a majority of the state delegations (the vote for President is state by state with each state getting one vote). Stein has no appeal to Never Trump Republicans, so she has no chance of forming a coalition that will win in the House.
Johnson is a more likely candidate. Johnson is a former two-term governor, so he has actual executive experience in government. If the election goes to the House, Johnson does have appeal to Never Trump Republicans. Also if Democrats are left with a decision between Trump and Johnson, their demonization of Trump would make it difficult for them to vote in a way that resulted in President Trump. This gives Johnson a narrow path to victory.
A lot of the problem is less about spending and more about infrastructure. Democrats and Republicans have an infrastructure of volunteers who can help with get-out-the-vote operations. Money is only a partial substitute for this.
Democrats and Republicans also have institutional inertia. There are people who've voted for whichever party for fifty years. Even if another candidate better represents their beliefs, they may not notice.