From the indispensable Open Secrets:

Outside groups have now spent more  than $400 million to influence elections this cycle, data collected from the Federal Election Commission show, a figure that dwarfs the amount spent by this point in the 2012 election.

These outside groups include not only super PACs and 527 organizations, which must disclose their donors, but also 501(c) nonprofit groups, which aren’t required to do so. Together, they’ve spent about $403 million — a 175 percent increase over the almost $146 million such groups had spent by this time in June 2012.

Not surprisingly, super PACs have driven the spending. With about $341 million in independent expenditures so far, they account for about 84 percent of all outside spending — roughly the same share as at this point in the last presidential cycle. Then, super PAC spending made up 81 percent of all political expenditures by outside groups.