Suppose you are in charge of a large political campaign (like, say, the ones for Obama and McCain that are going on the moment here in the US). You have a certain amount of money to spend and want to spend that money in a way that will make as many people vote for your candidate as possible. As always with such things, there are bound to be more things that you can conceive of doing than there is available money, so you have to choose only the things that meet a certain "expected number of voters swung per dollar" threshold. I wonder what that threshold is? I.e. if you give $100 to a political campaign, how many extra votes does that buy?
This isn't the same as the total number of votes cast for a candidate divided by the total amount that campaign spent, because some of those voters would have voted for that candidate anyway without the campaign spending any money. I'm interested in the marginal cost of a vote.
I'm sure the figure varies from day to day (if the other candidate makes a big gaffe, you can probably exploit that to swing a lot of voters relatively cheaply) and from state to state (votes in swing states are more valuable than votes in safe states, so it's worth spending more to swing them). I expect it also varies depending on how much the other campaign spends (since it costs money to undo their work). I'm sure the political campaigns do calculations to figure this stuff out - it would be interesting to see their statistics.