The amount of money that Presidential candidates spend on their campaigns is just mind boggling to me. Chances are good that it's mind boggling to your students too. CNN's Explain It To Me series has a couple of videos explaining how candidates raise money and what they spend that money on.
Here is How Candidates Raise Money.
Here is Campaign Spending. One of the statistics in this video that might help your students understand how much money is spent on campaigns is that Kobe Bryant would have to play for 29 years to earn as much money as President Obama spent in 2008.
On a much lighter note, and use your discretion before using this in your classroom, the Colbert Super Pac Coordination Resolution With Jon Stewart actually does include some good explanation of a couple of finer points about political super pacs.