The argument that online ads will generally fare well in a recession usually goes something like this: online advertising cheaper than traditional print and television advertising and offers far more accurate ROI measurement, so when budgets are squeezed, Internet advertising will look more attractive. "The thing we could well see is, a recession could expedite the shift from traditional spending to digital spending," said Jeremy Wright, global director of mobile brand strategy at Nokia Interactive, at Ad:Tech last month.

But a new report from PubMatic appears to indicate otherwise. Their May AdPrice Index, which was prepared by independent statisticians Dr. Albert Madansky and Dr. Michele Madansky, indicates that ad prices are starting to drop.

The report found that ad prices (based on effective CPMs) in April across all sites fell an average of 23%. This was most acutely felt by large sites (over 100 million page views per month), led by social networking sites, which saw eCPMs plummet 47% from March to April. Medium-sized web site monetization was essentially flat, while small sites (less than 1 million page views per month) saw modest gains month-over-month.

Social networking eCPMs sit at 19 cents, according to the AdPrice Index report, below January lows of 22 cents. The technology sector was basically flat from month-to-month, but still well off beginning of the year highs.

This all could indicate that a general US economic downturn is starting to be felt on the web. While the study didn't look specifically at search ads -- which analysts have said would be the last to feel the pain of a recession -- and it didn't differentiate between display and text ads, or between eCPMs from ad network to ad network, it is a general indicator of a slow down in the online ad market. Granted, this is only a couple of months of data, so it would be hard to create concrete trend predictions from it.

PubMatic's AdPrice Index is made up of over 3,000 web sites, about 85% of which are based in the US.