A short WebmasterWorld thread has some good nuggets on the level of traffic you should expect from a normal Yahoo! Search Marketing campaign relative to a Google AdWords campaign.
WebmasterWorld member, WebFusion, posted his confusion over just under 3% of the traffic he gets from Google with his Yahoo paid campaign. He explained how he used the exact same campaign, keywords, ads, ad copy and so on that he is using at Google AdWords plus he is bidding more at Yahoo. The ads have the same position in the search results as well. But he only received 25 visits per day from Yahoo, but he received 750 a day from Google.
Both shorebreak and Skibum, a WebmasterWorld moderator (Elis), explain that what WebFusion is seeing, is not the norm. Typically, Yahoo should send you about 20 - 25% of the traffic Google is sending you. But this can vary widely by industry. Skibum explains;
2% is about what one would expect for Ask.com give or take a percent or two. MSN around 10% and like ShoreBreak said, around 20 - 25% for yahoo! but it can vary widely by industry.
I would love to see more feedback in that thread and if you noted what industry it was, that would be even better. I know that organically, Ask.com send me more traffic than Yahoo did this past month - and we are included in Yahoo! News.
Forum discussion at WebmasterWorld.