The best example, until now, has been the re-election of Barack Obama in 2012. Big money did everything it could to stop him. But it couldn't erase Romney's thoughtless comment about the "47%" not mattering, and the video recording of that comment likely helped him lose the election. YouTube beat big money.
This year, Bernie Sanders is doing pretty well - explicitly denouncing the role of money in politics provided by corporate giants. Clearly, if that money were so unstoppable in its influence, Sanders wouldn't be where he is today.
And a statistic just released today shows another way in which money has had little if any lasting influence in the 2016 Presidential campaign thus far. As indicated in the chart, shown earlier on MSNBC, Jeb Bush has far and away spent the most per vote received by any GOP candidate - which means he has spent the most amount of money, and received the last in return in voter support. Conversely, Trump has spend the least - and his low expenditures have given him a second place finish in Iowa in a crowed field.
Clearly, then, money is not a decisive factor. Social media are apparently more decisive. In Trump's case, his domination of Twitter brought him to second place (and the fact that it was second, not first, shows that social media have their limits, too - see here for more).
I've always thought that, as John Milton and Thomas Jefferson pointed out, when truth and falsity are given access in the marketplace of ideas, people will recognize the truth and vote their self-interests. This is where social media find their limits, too - human rationality can recognize a lie, or bombast such as Trump's, whether presented in a high-cost ad or in a no-cost Tweet. But the advent of social media means that, regardless of how much money may be spent to the contrary, someone, somewhere, will be able to express a fact or a view closer to the truth. And with the mass media picking up more from social media, that fact will sooner or later be broadcast to millions.