I've been reading Big Money by Kenneth P. Vogel. The book talks about the rise of Super PACs and big money in politics.
I'll start by saying big money has always been around in politics, and it doesn't always help. I was in Minnesota in 1982 when Mark Dayton ran for US Senate, spending $7 million of his own money (a record for any senate campaign at the time) and losing.
What struck me is thinking about the cost per vote of an election. A total of $7 billion was spent by he candidates, parties, and other grups in the 2012 presidential election. So what is the cost per vote?
About 127 million votes were cast. This means a over $50.00 was spent per vote cast, quite a sum. However, since most votes are pre-determined before the election (a large number of voters automatically vote for their party of choice and aren't going to cross party lines) so the real cost per vote must consider the number of votes which might be swayed. This also includes the number of people are convinced to go vote who wouldn't otherwise vote, along with the number who are convinced not to vote who might otherwise vote (this being money spent by the other side to "inhibit" a vote).
My guess is that, barring a candidate with extreme charisma (e.g. Ronald Reagan getting working class democratic votes) at most 10% of voters are subject to persuasion. So this implies that roughly 10-12 million voters are the target of all the political advertising, recorded phone calls, etc.
Given this smaller number, for a major party candidate, the actual cost per vote of a presidential election is more on the order of $500.