The increases in our revenues reflect growth in the volume of lead referrals to merchants and other listings providers, which rose from 67.7 million in the three months ended March 31, 2004 to 82.3 million in the three months ended March 31, 2005, and average revenues per lead referral of $0.286 and $0.332 for the three months ended March 31, 2004 and 2005, respectively
According to MarketWatch [marketwatch.com] (which is by no means always accurate)
.......At the same time, 45% of the comparison-shopping engines' revenue comes from Google.
AdSense may very well be classified as "lead referrals" cause it's not banners and Shopping.com has some of its own sponsorships. If there isn't content in Shopping.com to match a search, AdWords always shows something.