From The Times:

The entry of Michael Bloomberg, the Mayor of New York, as an independent candidate could see election spending for 2008 move towards $5 billion – compared with £50 million for the British general election in 2005.

Jan Baran, a Washington lawyer who specialises in campaign finance, said that the open contests for both party nominations – and intensity with which they are being fought – meant that spending would smash all previous records.

Full Article:
http://www.timesonline.co.uk/tol/news/world/us_and_americas/us_elections/article3168726.ece