According to a press release by ColorOfChange.org (emphasis added):

The advertising boycott of Glenn Beck has cost the controversial host over half of his estimated advertising revenue since it was launched by ColorOfChange.org a month ago. This according to data analyzed from industry sources.

Estimated advertising revenue [the total amount of advertising money being spent during a block of commercial time for a program] was collected on a week-by-week basis for a period of two months. According to the data collected, the amount of money spent by national advertisers on Beck's program per week was at its highest at approximately $1,060,000, for the week ending August 2, 2009. ColorOfChange.org launched their campaign at the end of that week and since then, 62 advertisers have distanced themselves from Beck. Data collected for the week ending September 6, 2009 shows Beck's estimated ad revenue at $492,000, equal to a loss of $568,000.

[...]

ColorOfChange.org launched its campaign against Beck last month after the Fox News Channel host called President Obama a “racist” who “has a deep-seated hatred for white people” during an appearance on Fox & Friends, remarks that they say indicates a pattern of race-baiting and fear-mongering on Beck's part. ColorOfChange.org called on its members to sign a petition urging companies who advertise on Glenn Beck to cut off their advertising support of his work. To date, over 180,000 people have signed the petition and 62 companies have pledged to not run any additional advertisements during Glenn Beck's show.

Previously:

Six new advertisers reportedly join Beck boycott, brings total to 62

Why won't the press tell the truth about the sweeping Glenn Beck ad boycott?

Glenn Beck's “deep-seated hatred” of Obama and why Fox News won't budge