The Center for Responsive Politics estimates that the cost of #Election2012 will approach $6B:
USA Today Article on Cost of #Election2012
Another analysis projects the cost closer to $10B:
Adage / John Shelton (STRATA) Commentary on Cost of Election 2012
A substantial portion of this expenditure is for TV ads, on which $1B has already been spent in the first half of 2012.
For perspective, the most expensive movie ever made, "Pirates of the Caribbean: At World's End" cost a mere $300M!
The expenditures for this year's election are widely attributed to the U.S. Supreme Court's ruling on Citizens United v. the Federal Election Commission, which cleared the way for super PACs to work with unattributed contributions:
Citizens United v. Federal Election Commission (wikipedia)
There's no question about it: this election will be the costliest ever... but I assert that the costs mentioned above are actually only a drop in the bucket, and the reasons for the high cost are not as simple as a single Supreme Court ruling. The political impasse of the past few years has had a substantially negative effect on American growth, as unresolved fiscal issues come home to roost during election season. The core issues - healthcare, social security, the debt ceiling, taxation, and government spending in general - have split American politics right down the middle.
As the campaigns debate massive fiscal policy shifts, several trillion dollars of market investment is sidelined by perceived risk. Under this prevailing uncertainty, business growth is suppressed, and expenses associated with high unemployment continue to accumulate. If we look at unemployment benefits alone, the expenses is of order ~$100B per year.
Taking the broader effects into account, this really could be the world's first trillion dollar election season. And unfortunately it won't end in November: based on current polling, it seems that neither side will emerge with a clear mandate for action on the core fiscal issues. Electorally speaking, this election could be as close as Bush vs. Gore.