The 2008 presidential election was the most expensive in US history. The candidates spent approximately $1.4 billion on their campaigns, with outside groups dumping in another $215 million, says data from the Center for Responsive Politics (opensecrets.org).
This year's contest will probably break that record. And the cash is going to flow for Congressional races, too. Outside spending will be one of the major factors.
A Politico article says that right-leaning independent groups plan to spend $1 billion on the 2012 elections. To be clear, the groups are looking at the Congressional and Senate races, too, not just the presidential contest; the article doesn't say how that many may get divvied up. Still, that's substantially higher than the $301.7 million worth of outside spending in the 2008 presidential and Congressional races.
In general, each presidential election costs more than the one before it. But outside spending grew at a higher rate during the 2004 and 2008 contests. And those elections predated the Supreme Court's Citizens United decision. The ruling allowed corporations - and unions by extension - to fund ads and campaigns supporting or opposing specific candidates; direct contributions to candidates are still forbidden.
The decision's effects have already been felt in Congressional elections. Consider these figures from opensecrets.org: