SAI has stats on online
Ad rates and their relative shifts . Overall its an 11drop, despite large gains in some verticals (I've copied the graphic below):
Takeaway - online ads are shifting to online quality audiences (ie those with money) - look at how News and Reference have risen, SocNets, Young Adults and Entertainment have dropped. One of the things that has intrigued me about Planet Adland in recent years is the relentless chase after yoof when the money is with the boomers (and internet penetration is now high enough to belie the argument that only the young use the 'Net.)
Apparently this is because (I am told) Ad agency people either are young, or still want to be, and don't like doing Ads to older people. I suspect this will change over the Crunch years, and may be a permanent structural change in the industry after that.