According to AdGooroo, the cost per thousand impressions (CPM) on the Yahoo! Bing network is 4 to 11 times cheaper than Google Adwords, for Q3 2012.
This huge price difference may be explained because Adwords is the first choice for advertisers with PPC campaigns, and its superior click-through rates (2-5 times higher CTR on Adwords than Yahoo! Bing).
The CPM for Yahoo! Bing network was $4.93 to $15.26 compared to the CPM on the Adwords network of $34.29 to $101.76.