PubMatic released a report that suggests online ad spend is impacted by the US economic woes, reports BizJournals.

Based on data from 3,000 affiliate ad publishers, PubMatic's second annual AdPrice Index shows online ad spend is down 23 percent on average.

The average CPM fell from $.40 in March to $.33 in April. Most of the drop was absorbed by larger sites, those with over 100 million monthly page views. Prices there dropped 52 percent from March to April, from $.38 to $.18. Some sites have tried spreading risk by distributing inventory across multiple ad networks.

(For a chart of the data, see coverage by MarketingCharts.)

Mid-tier sites, those with 100 million to one million monthly views, were more or less steady, with CPMs declining to $.33 from $.34 last year. Smaller sites saw CPMs rise from $1.17 to $1.29.

With regard to verticals, the biggest drop hit entertainment sites, which saw 17 percent decreases. Technology sites fared best, with average CPM rising from $.82 in March to $.83 in April.