NBC has an interesting summary of ad money spent so far.
On the GOP side 95% of the money has been spent by outside groups and the vast majority of it has been for the four 'mainsteam' candidates (Bush, Rubio, Kasich and Christie). Bush's Super PAC is by far the biggest spender with $15.5M. As for the front-runners, Trump has spent $0 and Carson's campaign has spent about $1M.
On the Dem side Hillary's campaign is the big spender with $6.5M, however the article notes this important difference...
Full article here:
http://www.nbcnews.com/meet-the-press/super-pacs-outside-groups-dominate-gop-16-ad-spending-n457351
